MTR raises default wait_for_pos_timeout from 300 to 1500 when tests
are run with valgrind. The same needs to be done for other
replication-related waits.
The change should fix one of failures mentioned in MDEV-10653
(rpl.rpl_parallel fails in buildbot with timeout), the one
on the valgrind builder; but not all of them
MASTER_GTID_WAIT() is similar to MASTER_POS_WAIT(), but works with a
GTID position rather than an old-style filename/offset.
@@LAST_GTID gives the GTID assigned to the last transaction written
into the binlog.
Together, the two can be used by applications to obtain the GTID of
an update on the master, and then do a MASTER_GTID_WAIT() for that
position on any read slave where it is important to get results that
are caught up with the master at least to the point of the update.
The implementation of MASTER_GTID_WAIT() is implemented in a way
that tries to minimise the performance impact on the SQL threads,
even in the presense of many waiters on single GTID positions (as
from @@LAST_GTID).
