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.
- Add first basic mysql-test-run test case which tests switch to new master
using MASTER_GTID_POS=AUTO.
- When we connect with GTID, do not use any old relay logs, as they may
contain the wrong events or be corrupt after crash.
- Fix old bug that fails replication if we receive a heartbeat event
immediately after an event was omitted in the stream from the master.
- Fix rpl_end to clear Gtid_Pos_Auto, to keep check_testcase happy.