Thanks to MDEV-12699, the doublewrite buffer will only be needed in
those cases when a page is being updated in the data file. If the page
had never been written to the data file since it was initialized,
then recovery will be able to reconstruct the page based solely on
the contents of the redo log files.

The doublewrite buffer is only really needed when recovery needs to read
the page in order to apply redo log.

Note: As noted in MDEV-19739, we cannot safely disable the doublewrite
buffer if any MLOG_INDEX_LOAD records were written in the past or will
be written in the future. These records denote that redo logging was
disabled for some pages in a tablespace. Ideally, we would have
the setting innodb_log_optimize_ddl=OFF by default, and would not allow
it to be set while the server is running. If we wanted to make this
safe, assignments with SET GLOBAL innodb_log_optimize_ddl=...
should not only issue a redo log checkpoint (including a write of all
dirty pages from the entire buffer pool), but it should also wait for
all pending ALTER TABLE activity to complete. We elect not to do this.

Avoiding unnecessary use of the doublewrite buffer should improve the
write performance of InnoDB.

buf_page_t::init_on_flush: A new flag to indicate whether it is safe to
skip doublewrite buffering when writing the page.

fsp_init_file_page(): When writing a MLOG_INIT_FILE_PAGE2 record,
set the init_on_flush flag if innodb_log_optimize_ddl=OFF.
This is the only function that writes that log record.

buf_flush_write_block_low(): Skip doublewrite if init_on_flush is set.

fil_aio_wait(): Clear init_on_flush.

recv_apply_hashed_log_recs(): Refer directly to recs.last->end_lsn
instead of iterating the entire list starting from recs.log.

when plugins_are_initialized we already called unlink_thd()
and that calls server_threads.erase()

InnoDB crash recovery buffers redo log records in a hash table.
The function recv_read_in_area() would pick a random hash bucket
and then try to submit read requests for a few nearby pages.
Let us replace the recv_sys.addr_hash with a std::map, which will
automatically be iterated in sorted order.

recv_sys_t::pages: Replaces recv_sys_t::addr_hash, recv_sys_t::n_addrs.

recv_sys_t::recs: Replaces most of recv_addr_t.

recv_t: Encapsulate a raw singly-linked list of records. This reduces
overhead compared to std::forward_list. Storage and cache overhead,
because the next-element pointer also points to the data payload.
Processing overhead, because recv_sys_t::recs_t::last will point to
the last record, so that recv_sys_t::add() can append straight to the
end of the list.

RECV_PROCESSED, RECV_DISCARDED: Remove. When a page is fully processed,
it will be deleted from recv_sys.pages.

recv_sys_t::trim(): Replaces recv_addr_trim().

recv_sys_t::add(): Use page_id_t for identifying pages.

recv_fold(), recv_hash(), recv_get_fil_addr_struct(): Remove.

recv_read_in_area(): Simplify the iteration.

- Field::compatible_field_size(), and its "Relay_log_info *rli" argument
- Field::pack_length_from_metadata

This patch includes:
- MDEV-19639 sql_mode=ORACLE: Wrong SHOW PROCEDURE output for sysvar:=expr
- MDEV-19640 Wrong SHOW PROCEDURE output for SET GLOBAL sysvar1=expr, sysvar2=expr
- Preparatory refactoring for MySQL WL#4179

Detailed change list:

1. Changing sp_create_assignment_lex() to accept the position
   in the exact query buffer instead of a "bool no_lookahead".
   This actually fixes MDEV-19639.
   In the previous reduction sp_create_assignment_lex() was
   called too late, when the parser went far the from beginning
   of the statement, so only a part of the statement got into
   sp_instr_stmt.

2. Generating "SET" or "SET GLOBAL" inside sp_create_assignment_instr()
   depending on the option type.
   This fixes MDEV-19640.
   In the previous reduction the code passed (through no_lookahead)
   the position of the
   word GLOBAL inside sp_create_assignment_lex(), which
   worked only for the left-most assignment.

3. Fixing the affected rules to use:
   - ident_cli instead of ident
   - ident_cli_set_usual_case instead of ident_set_usual_case

4. Changing the input parameter in:
   - LEX::set_system_variable()
   - LEX::call_statement_start()
   - LEX::set_variable()
   from just LEX_CSTRING to Lex_ident_sys_st for stricter data type constrol:
   to make sure that noone passes an ident_cli
   (a fragment of the original query in the client character set)
   instead of server-side identifier
   (utf8 identifier allocated on THD when needed).

5. Adding Lex_ident_sys() in places where the affected functions are called.

6. Moving all calls of sp_create_assignment_lex() to the places
   just before parsing set_expr_or_default.
   This makes the grammar clearer, because
   sp_create_assignment_lex() and sp_create_assignment_instr()
   now stay near each other, so the balance of LEX's push/pop
   can be read easier.
   This will also help to WL#4179.

7. Adding class sp_lex_set_var
   Moving the initialization code from
   sp_create_assignment_lex() to the constructor of sp_lex_set_var.
   This will also help to WL#4179.

8. Moving a part of the "set" grammar rule into a separate
   rule "set_param".
   This makes the grammar easier to read and removes
   one shift/reduce conflict.

client-certkey.pem contains both certificate and corresponding
private key.

This removes dependency of libhmash

A thread_pool_stall_limit which is smaller than default would result
in quicker creation of threads.

row_search_mvcc(): Duplicate the logic of btr_pcur_move_to_next()
so that an infinite loop can be avoided when advancing to the next
page fails due to a corrupted page.

Also, --skip-innodb-buffer-pool-load-at-startup to avoid a crash
in buf_load() due to loading pages that we are corrupting intentionally.

Apprently, sometimes there will be null pointers with 0 length
passed to the MyCTX::update() function, and  will need to return
a valid buffer.

So weaken the assertion, and use a valid pointer for src if it was NULL.

At higher levels of innodb_force_recovery, the InnoDB transaction
subsystem will not be set up at all.

At slightly lower levels, recovered transactions will not be rolled back,
and DDL operations could hang due to locks being held at all.

Let us consistently refuse all writes if the predicate
high_level_read_only holds. We failed to refuse DROP TABLE
and DROP DATABASE. (Refusing DROP TABLE is a partial backport
from MDEV-19570 in the 10.5 branch.)

- Replace mysql_version check with omit_virtual_cols() in
ha_innobase::check_if_supported_inplace_alter().

- If one of the encryption threads already started the initialization
of the tablespace then don't remove the other uninitialized tablespace
from the rotation list.

- If there is a change in innodb_encrypt_tables then
don't remove the processed tablespace from rotation list.

LOCK_global_system_variables: Declare with MYSQL_PLUGIN_IMPORT

Changed the function append_range_all_keyparts to use sel_arg_range_seq_init / sel_arg_range_seq_next to produce ranges.
Also adjusted to print format for the ranges, now the ranges are printed as:
    (keypart1_min, keypart2_min,..)  OP (keypart1_name,keypart2_name, ..) OP (keypart1_max,keypart2_max, ..)

Also added more tests for range and index merge access for optimizer trace