Put unhandled responses back on the queue

[mjg59]

In a multithreaded environment (such as having a separate thread to wait
for notifications), it's possible that the response for one event (such
as the completion of a write) may be consumed by another thread (such as
one that's just waiting for notifications). If the write response is never
delivered to the correct thread, it will block - and if there are no
timeouts, it will deadlock.

This approach simply makes the assumption that if an unhandled event shows
up, it was probably intended for another thread - simply putting it back
on the queue will allow that other thread to have a go at parsing it. In
the event of a genuinely unexpected message this will result in a lot of
busy work and the event never being cleared, which isn't ideal. I'm not
sure if there's a more reasonable approach to this.

[mjg59]

I think this should fix #349