Fix replication for multi-update
  new test - rpl_multi_update2


sql/mysql_priv.h:
  Bug#7011
    New function mysql_multi_update_lock()
sql/sql_parse.cc:
  Bug#7011
    New function check_multi_update_lock()
    For multi-update on slave, perform an early open&lock
sql/sql_update.cc:
  Bug#7011
    Split out multi-update locking into its own function,
      mysql_multi_update_lock()