Bug#319  if while a non-transactional slave is replicating a transaction possible problem 

It is impossible to roll back a mixed engines transaction when one of the engine is
non-transaction. In replication that fact is crucial because the slave can not safely
re-apply a transction that was interrupted with STOP SLAVE.

Fixed with making STOP SLAVE not be effective immediately in the case the current
group of replication events has modified a non-transaction table. In order for slave to leave
either the group needs finishing or the user issues KILL QUERY|CONNECTION slave_thread_id.