Per-engine mysql.gtid_slave_pos tables

Implement that server reads/updates the GTID position using multiple tables
mysql.gtid_slave_pos_XXX, each using a different storage engine. Replicated
transactions update the version of the table in the same engine, if
available. This avoids the overhead of cross-engine transactions on servers
where multiple storage engines are in use at the same time (but not in the
same transactions).

See mailing list thread: https://lists.launchpad.net/maria-developers/msg10453.html