This happend when using XA transactions. I also added some extra asserts
to ensure that m_transactions are properly cleared.

Other things:
- Removed set_time() from THD::init_for_queries() as dispatch_command()
  is already doing that.
- Removed duplicate init_for_queries() from prepare_new_connection_state().
  The init_for_queries() functions should only be called once per
  connection.

This patch ensures that all identical character sets shares the same
cs->csname.
This allows us to replace strcmp() in my_charset_same() with comparisons
of pointers. This fixes a long standing performance issue that could cause
as strcmp() for every item sent trough the protocol class to the end user.

One consequence of this patch is that we don't allow one to add a character
definition in the Index.xml file that changes the csname of an existing
character set. This is by design as changing character set names of existing
ones is extremely dangerous, especially as some storage engines just records
character set numbers.

As we now have a hash over character set's csname, we can in the future
use that for faster access to a specific character set. This could be done
by changing the hash to non unique and use the hash to find the next
character set with same csname.

The problem was that field_count is not initialized for the Protocol
variable used when printing metadata.

- Better to use 'String *' directly.
- Added String::get_value(LEX_STRING*) for the few cases where we want to
  convert a String to LEX_CSTRING.

Other things:
- Use StringBuffer for some functions to avoid mallocs

- Should speed up replication

- Removed val_str() and print() as these are handled by Item_int()
- Use local StringBuffer for Item_int::print() to avoid mallocs

Starting with MDEV-17441 we would no longer have os_once,
and we would always initialize zip_pad_info_t::mutex and
dict_table_t::autoinc_mutex, even for tables are not in
ROW_FORMAT=COMPRESSED nor include any AUTO_INCREMENT column.

mutex_free() on those unnecessary objects would make shutdown very slow
compared to older versions.

Let us use std::mutex for those two mutexes, to reduce the overhead.
The critical sections protected by these mutexes is very small, and
therefore contention or the need for any instrumentation should
be unlikely.

If there is no current_select and variable is not found among SP variables it can be only an error.

MDEV-23252 Assertion failure 'req_type.is_dblwr_recover() || err == DB_SUCCESS' for page_compressed tables

- This issue is caused by a5584b13
(MDEV-15528). os_file_punch_hole() is added to fil_io() in MDEV-15528.
But it fails to handle failure of os_file_punch_hole(). InnoDB should
handle the DB_IO_NO_PUNCH_HOLE error and silently transform to
DB_SUCCESS. InnoDB should set the punch hole flag correctly when
tablespace is loaded

fil_node_t::read_page0(): Set the punch hole flag when tablespace is loaded

fil_io(): Handle the DB_IO_NO_PUNCH_HOLE error

buf_flush_free_pages(): Checks the punch hole condition earlier using
tablespace punch hole flag

InnoDB should replace FSP_FLAGS_HAS_PAGE_COMPRESSION check with
fil_space_t::is_compressed(). fil_space_t::is_compressed() checks
for both non full crc32 and crc32 format.

InnoDB should replace FSP_FLAGS_HAS_PAGE_COMPRESSION check with
fil_space_t::is_compressed(). fil_space_t::is_compressed() checks
for both non full crc32 and crc32 format.

When high priority replication slave applier encounters lock conflict in innodb,
it will force the conflicting lock holder transaction (victim) to rollback.
This is a must in multi-master sychronous replication model to avoid cluster lock-up.
This high priority victim abort (aka "brute force" (BF) abort), is started
from innodb lock manager while holding the victim's transaction's (trx) mutex.
Depending on the execution state of the victim transaction, it may happen that the
BF abort will call for THD::awake() to wake up the victim transaction for the rollback.
Now, if BF abort requires THD::awake() to be called, then the applier thread executed
locking protocol of: victim trx mutex -> victim THD::LOCK_thd_data

If, at the same time another DBMS super user issues KILL command to abort the same victim,
it will execute locking protocol of: victim THD::LOCK_thd_data  -> victim trx mutex.
These two locking protocol acquire mutexes in opposite order, hence unresolvable mutex locking
deadlock may occur.

The fix in this commit adds THD::wsrep_aborter flag to synchronize who can kill the victim
This flag is set both when BF is called for from innodb and by KILL command.
Either path of victim killing will bail out if victim's wsrep_killed is already
set to avoid mutex conflicts with the other aborter execution. THD::wsrep_aborter
records the aborter THD's ID. This is needed to preserve the right to kill
the victim from different locations for the same aborter thread.
It is also good error logging, to see who is reponsible for the abort.

A new test case was added in galera.galera_bf_kill_debug.test for scenario where
wsrep applier thread and manual KILL command try to kill same idle victim

trx_update_mod_tables_timestamp(): When implementing
innodb_evict_tables_on_commit_debug, do not evict tables
on which transactional locks exist.

This debug variable was broken since its introduction in
commit 947b0b57.

Problem was that the code didn't handle a transaction created in innodb
as part of a failed mysql_lock_tables()

The issue was:
T1, a parallel slave worker thread, is waiting for another worker thread to
commit. While waiting, it has the MDL_BACKUP_COMMIT lock.
T2, working for mariabackup, is doing BACKUP STAGE BLOCK_COMMIT and blocks
all commits.
This causes a deadlock as the thread T1 is waiting for can't commit.

Fixed by moving locking of MDL_BACKUP_COMMIT from ha_commit_trans() to
commit_one_phase_2()

Other things:
- Added a new argument to ha_comit_one_phase() to signal if the
  transaction was a write transaction.
- Ensured that ha_maria::implicit_commit() is always called under
  MDL_BACKUP_COMMIT. This code is not needed in 10.5
- Ensure that MDL_Request values 'type' and 'ticket' are always
  initialized. This makes it easier to check the state of the MDL_Request.
- Moved thd->store_globals() earlier in handle_rpl_parallel_thread() as
  thd->init_for_queries() could use a MDL that could crash if store_globals
  where not called.
- Don't call ha_enable_transactions() in THD::init_for_queries() as this
  is both slow (uses MDL locks) and not needed.

MDEV-22899 Assertion `field->col->is_binary() || field->prefix_len % field->col->mbmaxlen == 0' failed in dict_index_add_to_cache

is_part_of_a_key(): detect is TEXT field is a part of some key

ha_innobase::can_convert_blob(): now correctly detect whether our blob
is a part of some key. Previously the check didn't work in some cases.

Fix stale virtual field value in 4 cases: when virtual field depends
on row_start/row_end in timestamp/trx_id versioned table. row_start
dep is recalculated in vers_update_fields() (SQL and InnoDB
layer). row_end dep is recalculated on history row insert.

MDEV-22061 InnoDB: Assertion of missing row in sec index row_start upon REPLACE on a system-versioned table

make_versioned_helper() appended new update field unconditionally
while it should check if this field already exists in update vector.

Misc renames to conform versioning prefix. vers_update_fields() name
conforms with sql layer TABLE::vers_update_fields().

           when scrubbing is enabled

buf_read_recv_pages(): Ignore the page to read if it is already
present in the freed ranges.

store_freed_or_init_rec(): Store the ranges only if scrubbing
is enabled or page compressed tablespace.

recv_init_crash_recovery_space(): Add the freed range only when
scrubbing or page compressed tablespace.

range_set::contains(): Search the value is present in ranges.

range_set::remove_if_exists(): Remove the value if exist in ranges.

mtr_t::init(): Handles the scenario that mini-transaction may allocate
a page that had just been freed.

recv_sys_t::parse(): Note down the FREE and INIT redo log irrespective
of STORE value.

Removed innodb_tablespaces_scrubbing from test case

When InnoDB is extending a data file, it is updating the FSP_SIZE
field in the first page of the data file.

In commit 8451e090 (MDEV-11556)
we removed a work-around for this bug and made recovery stricter,
by making it track changes to FSP_SIZE via redo log records, and
extend the data files before any changes are being applied to them.

It turns out that the function fsp_fill_free_list() is not crash-safe
with respect to this when it is initializing the change buffer bitmap
page (page 1, or generally, N*innodb_page_size+1). It uses a separate
mini-transaction that is committed (and will be written to the redo
log file) before the mini-transaction that actually extended the data
file. Hence, recovery can observe a reference to a page that is
beyond the current end of the data file.

fsp_fill_free_list(): Initialize the change buffer bitmap page in
the same mini-transaction.

The rest of the changes are fixing a bug that the use of the separate
mini-transaction was attempting to work around. Namely, we must ensure
that no other thread will access the change buffer bitmap page before
our mini-transaction has been committed and all page latches have been
released.

That is, for read-ahead as well as neighbour flushing, we must avoid
accessing pages that might not yet be durably part of the tablespace.

fil_space_t::committed_size: The size of the tablespace
as persisted by mtr_commit().

fil_space_t::max_page_number_for_io(): Limit the highest page
number for I/O batches to committed_size.

MTR_MEMO_SPACE_X_LOCK: Replaces MTR_MEMO_X_LOCK for fil_space_t::latch.

mtr_x_space_lock(): Replaces mtr_x_lock() for fil_space_t::latch.

mtr_memo_slot_release_func(): When releasing MTR_MEMO_SPACE_X_LOCK,
copy space->size to space->committed_size. In this way, read-ahead
or flushing will never be invoked on pages that do not yet exist
according to FSP_SIZE.

commit 854c219a (MDEV-17301)
broke a constraint: Fixed-length columns cannot be extended in InnoDB
without rebuilding the table.

ha_innobase::can_convert_string(): Correct the condition. We must
not allow any instantaneous change to the length of CHAR columns
measured in characters. For any format other than ROW_FORMAT=REDUNDANT,
we can allow the length in bytes to be extended if mbminlen<mbmaxlen held
before the change of the character set.

m_file[0] not always is a good sample.