invocations of LAST_INSERT_ID.

Reding of LAST_INSERT_ID inside stored function wasn't noted by caller,
and no LAST_INSERT_ID_EVENT was issued for binary log.

The solution is to add THD::last_insert_id_used_bin_log, which is much
like THD::last_insert_id_used, but is reset only for upper-level
statements.  This new variable is used to issue LAST_INSERT_ID_EVENT.