- 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.