FR: Consider showing GTID-based 'until' value in slave status

It can be difficult to understand what a slave is waiting for or why it is not replicating something if it replication was started as START SLAVE UNTIL gtid_master_pos – unlike traditional master position, the given list of GTIDs is not shown anywhere (as far as I can tell).

Since the list is slave-specific (not global for all slaves as current_pos or binlog_pos or slave_pos are), the good place for it would be in SHOW SLAVE STATUS. Sadly, it seems that MySQL didn't add it, so if we decide that we want it, we'll have to add it and further diverge from MySQL's SHOW SLAVE STATUS output.