that we can later use it to avoid unnecessary trim operations. 

The problem was when a GTID event was part of a group commit, and so contained
a commit id. The code that replaces GTID with a BEGIN event for old slaves did
not correctly handle this case.

Fix the code so that the GTID with commit id can also be properly replaced
with a BEGIN query event. The extra two bytes are in the BEGIN event replaced
with a dummy, empty time zone string.

Older master has no GTID events, so such events are not available for
deciding on scheduling of event groups and so on.

With this patch, we run such events from old masters single-threaded, in the
sql driver thread.

This seems better than trying to make the parallel code handle the data from
older masters; while possible, this would require a lot of testing (as well as
possibly some extra overhead in the scheduling of events), which hardly seems
worthwhile.

individual queue's. Tried to minimize memory allocation from heap
whenever it is unnecessary.

With parallel replication, there can be any number of events queued on
in-memory lists in the worker threads.

For normal STOP SLAVE, we want to skip executing any remaining events on those
lists and stop as quickly as possible.

However, for START SLAVE UNTIL, when the UNTIL position is reached in the SQL
driver thread, we must _not_ stop until all already queued events for the
workers have been executed - otherwise we would stop too early, before the
actual UNTIL position had been completely reached.

The code did not handle UNTIL correctly, stopping too early due to not
executing the queued events to completion. Fix this, and also implement that
an explicit STOP SLAVE in the middle (when the SQL driver thread has reached
the UNTIL position but the workers have not) _will_ cause an immediate stop.

MDEV-5748 Assertion `status_var.memory_used == 0' fails on disconnect after opening an OQGRAPH table

make sure MY_THREAD_SPECIFIC is not set for memroot that can be
transferred between threads

MDEV-5674 Valgrind warnings "Conditional jump or move depends on uninitialised value" in create_sort_index with small sort_buffer_size

*found_rows wasn't initialized when filesort() failed
(it didn't matter, but valgrind was unhappy)

use the Alter_inplace_info::ALTER_COLUMN_OPTION flag if
field/column flags were altered.

change ha_example to use check_if_supported_inplace_alter() instead
of obsolete check_if_incompatible_data()

parser: error out on empty role names

don't forget to initialize ORDER::fast_field_copier_setup

Added option to disable multi-threaded flush with innodb_use_mtflush = 0
option, by default multi-threaded flush is used.

Updated innochecksum tool, still it does not support new checksums.

Fix ha_table_exists() to take discovery into account correctly.
It must be able to discover both table existence (when no frm is
found) and table non-existance (when frm was found).

apply the upstream patch

Disable query cache for OQGRAPH

Add a check for binlog open already during the parsing, so that
BINLOG_GTID_POS() can return the error early and not delay it
until execution.

update test results