This is also related to
MDEV-31348 Assertion `last_key_entry >= end_pos' failed in virtual bool
           JOIN_CACHE_HASHED::put_record()

Valgrind exposed a problem with the join_cache for hash joins:
=25636== Conditional jump or move depends on uninitialised value(s)
==25636== at 0xA8FF4E: JOIN_CACHE_HASHED::init_hash_table()
          (sql_join_cache.cc:2901)

The reason for this was that avg_record_length contained a random value
if one had used SET optimizer_switch='optimize_join_buffer_size=off'.

This causes either 'random size' memory to be allocated (up to
join_buffer_size) which can increase memory usage or, if avg_record_length
is less than the row size, memory overwrites in thd->mem_root, which is
bad.

Fixed by setting avg_record_length in JOIN_CACHE_HASHED::init()
before it's used.

There is no test case for MDEV-31893 as valgrind of join_cache_notasan
checks that.
I added a test case for MDEV-31348.

# Conflicts:
#	mysql-test/main/sp.result
#	mysql-test/main/sp.test

Remove redundant delete_explain_query() calls in

sp_instr_set::exec_core(), sp_instr_set_row_field::exec_core(),
sp_instr_set_row_field_by_name::exec_core().

These calls are made before the SP instruction's tables are
"closed" by close_thread_tables() call.

When we call close_thread_tables() after that, we no longer
can collect engine's counter variables, as they use the data
structures that are located in the Explain Data Structures.

Also, these delete_explain_query() calls are redundant, as
sp_lex_keeper::reset_lex_and_exec_core() has another
delete_explain_query() call, which is located in the right
location after the close_thread_tables() call.

- InnoDB aborts when table is dropping the column. This is
caused by 5f09b53b (MDEV-31086).
While iterating the altered table fields, we fail to consider
the dropped columns.

when validating vcol's (default, check, etc) in ALTER TABLE
vcol_info->flags are modified in place. This means that if ALTER TABLE
fails for any reason we need to restore them to their original values.

(mroonga was freeing the memory on ::reset() but not on ::close())

`select * from information_schema.columns` can return columns in any
order, but we need them ordered by the ordinal position

MDEV-31743 Server crash in store_length, assertion failure in Type_handler_string_result::sort_length

After MDEV-21580 the truncation of SORT_FIELD::length
  set_if_smaller(sortorder->length, thd->variables.max_sort_length)

became conditional:
  if (is_variable_sized())
    set_if_smaller(length, thd->variables.max_sort_length)

To provide correct functioning of is_variable_sized() SORT_FIELD::type
must be set properly. This commit adds the necessary initialization
of SORT_FIELD::type to JOIN_TAB::remove_duplicates() as it is done
in filesort's sortlength() function.

DBUG_ASSERT is added to sortlength() just in case to prevent
a possible uint32 overflow

recv_scan_log(): On recv_sys_t::PREMATURE_EOF, keep reading more log
if recv_sys.lsn < recv_sys.scanned_lsn.

recv_recovery_from_checkpoint_start(): Add a safety check to abort
crash recovery if recv_sys.lsn is not recv_sys.scanned_lsn.

This fixes a serious database corruption bug that was introduced by
commit 2f9e2647 (MDEV-29911).

MDEV-31822 ALTER TABLE ENGINE=x started failing instead of producing warning on unsupported TRANSACTIONAL=1

make TRANSACTIONAL table option behave similar to other engine-defined
table options. If the engine doesn't suport it:
* if specified expicitly in CREATE or ALTER - it's ER_UNKNOWN_OPTION
* an error or a warning depending on sql_mode IGNORE_BAD_TABLE_OPTIONS
* in ALTER TABLE from the engine that suppors it to the engine that
  doesn't - silently preserved (no warning)
* it is commented out in SHOW CREATE unless IGNORE_BAD_TABLE_OPTIONS

* invoke check_expression() for all vcol_info's in
  mysql_prepare_create_table() to check for FK CASCADE
* also check for SET NULL and SET DEFAULT
* to check against existing FKs when a vcol is added in ALTER TABLE,
  old FKs must be added to the new_key_list just like other indexes are
* check columns recursively, if vcol1 references vcol2,
  flags of vcol2 must be taken into account
* remove check_table_name_processor(), put that logic under
  check_vcol_func_processor() to avoid walking the tree twice

recv_ring::copy_if_needed(): If the record wraps around the
memory-mapped ib_logfile0, do copy it also if len==0
(the record consists only of a header, like FREE_PAGE and INIT_PAGE
records do).

recv_sys_t::parse(): Invoke recv_ring::copy_if_needed() for INIT_PAGE
and FREE_PAGE records, so that if these records wrap around the
memory-mapped ib_logfile0, they will be correctly copied to
recv_sys.pages.

Together with commit 0d175968 (MDEV-31354)
this fixes occasional failures of the test innodb.recovery_memory.

mark old keys in the ALTER TABLE with the `old` flag, not with
the `key_create_info.check_for_duplicate_indexes`.

This allows to mark old foreign keys too.

to declutter Item_field::check_vcol_func_processor(), move alter_info
specific part of it into Alter_info::check_vcol_field()

also, prefer Lex_table_name and Lex_ident over LEX_CSTRING