This patch introduces a small optimization on the automatic GNO
assignment for automatic GTIDs.

The previous approach was to get the first non-committed GNO as a
starting point, and then to find the first GNO not owned by other
transactions that reached the flush stage.

The above described approach would result in the thread spending more
time than needed while holding the sidno_lock for the server UUID.

The optimization memorizes the last GNO assigned automatically and
starts looking for available GNOs from that point.

This patch also made Gtid_state::init to require global_sid_lock.wrlock
instead of global_sid_lock.rdlock.