Starting with commit 210855ce
Valgrind became aware that the unused tail of the buffer that
is returned by thd_get_xid() is actually uninitialized.

The problem should exist already in MySQL 5.0. I was able to
repeat it on MariaDB Server 5.5 with some additional instrumentation.
InnoDB is allocating 128+4+4 bytes for the XID and the lengths of
its components, even when the XID is shorter than 64+64 bytes.
In MariaDB Server 10.3, while running the test main.xa_binlog,
in the xid_t::set() that is called by sql_yacc.yy, the 128-byte data
buffer was uninitialized according to Valgrind, and only the first bytes
were initialized. When the xid_t::data was copied to
thd.transaction.xid_state.xid.data, it happened so that the entire
target buffer was considered initialized. With MariaDB Server 10.4 since
the said commit, Valgrind will correctly be detect the tail of the buffer
as uninitialized.

The impact of this bug is as follows:

(1) InnoDB will write unnecessarily much redo log for XA PREPARE.
(2) InnoDB will write garbage bytes to the redo log and undo log pages.
(3) The garbage should be 'harmless', because on recovery, only the
actual payload of the XID will be used, based on the written length.

trx_rseg_write_wsrep_checkpoint(), trx_undo_write_xid(): Write only
the actually used length of xid->data to the data page, and
zero out the rest of the buffer by mlog_memset().

This fixes the following test failures related to index cardinality:
main.join main.stat_tables main.partition main.stat_tables_innodb
innodb.innodb_bug57252

- Initialize variables that could be used uninitialized
- Added extra end space to DbugStringItemTypeValue to get rid of warnings
  from c_ptr()
- Session_sysvars_tracker::update() accessed unitialized memory if called
  with NULL value.
- get_schema_stat_record() accessed unitialized memory if HA_KEY_LONG_HASH
  was used
- parse_vcol_defs() accessed random memory for tables without keys.

MDEV-19301 Assertion `!is_valid_datetime() || fraction_remainder(((item->decimals) < (6) ? (item->decimals) : (6))) == 0' failed in Datetime_truncation_not_needed::Datetime_truncation_not_needed

Ignore the setreuid() return here.

Make sure that the references in the GROUP BY clause that need to be considered for
pushdown from having to where are of Item_field objects

Use LOCK_server_started, COND_server_started and mysqld_server_started for waiting starting the server

Do not cast wrong type.

We use own IO callbacks from server code anyway.

Fix reinitialize vers_write on table reopen.

Make all accesses to btr_cur_n_non_sea atomic.

Use the same select as for usual table list (forgotten oracle mode parser fixed)

fix is simple: just move mp->file under mp->global protection

MDEV-18350 Using audit plugin with MySQL, mysqld crashes when running COM_INIT_DB against invalid database.

mysql_event_general structure changed in 10.3.
the database_length should be size_t now instead of the int.

Turn off versioned_write for DELETE HISTORY.

* Made make_versioned_*() proxies inline;
* Renamed truncate_history to delete_history

Part of:
MDEV-19814 Server crash in row_upd_del_mark_clust_rec or Assertion
`update->n_fields < ulint(table->n_cols + table->n_v_cols)' failed in
upd_node_t::make_versioned_helper