Intermediate commit.

Move the discovery of mysql.gtid_slave_pos* tables into the SQL thread.

This avoids doing things like opening tables and scanning the mysql
schema for tables inside of the START SLAVE statement, which might
interact badly with existing transaction or table locks.

(Even though START SLAVE is documented to implicitly commit any active
transactions, this appears not to be the case in current code).

Table discovery fits naturally in the SQL thread init code, next to
the loading of mysql.gtid_slave_pos state.