Intermediate commit.

This commit implements that record_gtid() selects a gtid_slave_posXXX table
with a storage engine already in use by current transaction, if any.

The default table mysql.gtid_slave_pos is used if no match can be found on
storage engine, or for GTID position updates with no specific storage
engine.

Table discovery of mysql.gtid_slave_pos* happens on initial GTID state load
as well as on every START SLAVE. Some effort is made to make this possible
without additional locking. New tables are added using lock-free atomics.
Removing tables requires stopping all slaves first. A warning is given in
the error log when a table is removed but a non-stopped slave still has a
reference to it.

If multiple mysql.gtid_slave_posXXX tables with same storage engine exist,
one is chosen arbitrarily to be used, with a warning in the error log. GTID
data from all tables is still read, but only one among redundant tables with
same storage engine will be updated.