MDEV-4984: Implement MASTER_GTID_WAIT() and @@LAST_GTID.

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).

mysql-test/include/sync_with_master_gtid.inc

@@ -0,0 +1,47 @@
+# ==== Purpose ====
+#
+# Wait until the slave has reached a certain GTID position.
+# Similar to --sync_with_master, but using GTID instead of old-style
+# binlog file/offset coordinates.
+#
+#
+# ==== Usage ====
+#
+# --let $master_pos= `SELECT @@GLOBAL.gtid_binlog_pos`
+# [--let $slave_timeout= NUMBER]
+# [--let $rpl_debug= 1]
+# --source include/sync_with_master_gtid.inc
+#
+# Syncs slave to the specified GTID position.
+#
+# Must be called on the slave.
+#
+# Parameters:
+#   $master_pos
+#     The GTID position to sync to. Typically obtained from
+#      @@GLOBAL.gtid_binlog_pos on the master.
+#
+#   $slave_timeout
+#      Timeout in seconds. The default is 2 minutes.
+#
+#   $rpl_debug
+#      See include/rpl_init.inc
+
+--let $include_filename= sync_with_master_gtid.inc
+--source include/begin_include_file.inc
+
+let $_slave_timeout= $slave_timeout;
+if (!$_slave_timeout)
+{
+  let $_slave_timeout= 120;
+}
+
+--let $_result= `SELECT master_gtid_wait('$master_pos', $_slave_timeout)`
+if ($_result == -1)
+{
+  --let $_current_gtid_pos= `SELECT @@GLOBAL.gtid_slave_pos`
+  --die Timeout in master_gtid_wait('$master_pos', $_slave_timeout), current slave GTID position is: $_current_gtid_pos.
+}
+
+--let $include_filename= sync_with_master_gtid.inc
+--source include/end_include_file.inc