Fix to allow the binlog format to be changed between the LOCK and
UNLOCK tables, as well as under MIXED mode.


mysql-test/r/rpl_switch_stm_row_mixed.result:
  Result change
mysql-test/t/rpl_switch_stm_row_mixed.test:
  Adding test to see that binlog format can be changed when using
  LOCK/UNLOCK TABLES both under ROW format and MIXED format.
sql/log.cc:
  Removing pre-condition assertion since binlog can now be
  statement based.
sql/sql_class.cc:
  Adding code to always flush pending event regardless of the binlog
  format used. The only exception is if we are inside a stored routine,
  where the pending event is never flushed.