Replace CHANGE MASTER TO ... master_gtid_pos='xxx' with a new system
variable @@global.gtid_pos.
This is more logical; @@gtid_pos is global, not per-master, and it is not
affected by RESET SLAVE.
Also rename master_gtid_pos=AUTO to master_use_gtid=1, which again is more
logical.
More fixes for test failures in Buildbot:
- Do not run crashing test in Valgrind.
- FLUSH TABLES did not work to avoid errors about not closed tables when
crashing server. Suppress the messages instead.
- Rewrite multi-source test case to only start one pair of slave threads at a
time, to work-around the bug MDEV-4352.
Adjust full test suite to work with GTID.
Huge patch, mainly due to having to update .result file for all SHOW BINLOG
EVENTS and mysqlbinlog outputs, where the new GTID events pop up.
Everything was painstakingly checked to be still correct and valid .result
file updates.
Add test case demonstrating multi-source replication with GTID.
Test that we can move from being slave of two masters to be a slave
deeper down in the hierarchy (of a higher-up multi-source slave),
and vice versa.
