IO thread should expose its current GTID position

Master_info already has the field gtid_current_pos representing GTID of the last full transaction saved into relay log. It would be extremely useful to have this field exposed e.g. in the result of SHOW SLAVE STATUS.

This information will be useful in failover scenarios. Currently when master dies we can't know which of the slaves has all the latest transactions in their relay log until SQL thread executes all of them. But if Master_info::gtid_current_pos is visible we'll be able to know exactly what transactions each slave has and which GTID position it will be at when SQL thread catches up with all relay log events.