Intermediate commit.

Keep track of which mysql.gtid_slave_posXXX tables are available for each
engine, by searching for all tables in the mysql schema with names that
start with "gtid_slave_pos".

The list is computed at server start when the GTID position is loaded, and
it is re-computed on every START SLAVE command. This way, the DBA can
manually add a table for a new engine, and it will be automatically picked
up on next START SLAVE, so a full server restart is not needed.

The list is not yet actually used in the code.