Move replication specific assertion out of mdl

  /*

    Ensure that if we are trying to get an exclusive lock for a slave

    running parallel replication, then we are not blocked by another

    parallel slave thread that is not committed. This should never happen as

    the parallel replication scheduler should never schedule a DDL while

    DML's are still running.

  */

  DBUG_SLOW_ASSERT((mdl_request->type != MDL_INTENTION_EXCLUSIVE &&

                    mdl_request->type != MDL_EXCLUSIVE) ||

                   !(get_thd()->rgi_slave &&

                     get_thd()->rgi_slave->is_parallel_exec &&

                     lock->check_if_conflicting_replication_locks(this)));