MemoryChannel should push events back into channel on rollback

Currently the MemoryChannel (i.e. the "best effort" channel) transaction provider drops events if the consumer rolls back. In other words a sink that does:

tx = channel.getTransaction()
event = channel.take()
tx.rollback()

...loses the event. Instead, the event should be pushed back into the channel so it's redelivered. Ideally, it should be inserted at the end of the channel to be as close to totally ordered as possible. There is the question of what to do if the channel is at capacity.