clearing message handlers on status=idle loses async output

[minrk]

It is possible for output to arrive on IOPub after the status=idle message.
Currently, we clear message handlers on the idle message, in order to keep that data structure from growing indefinitely, but the result is lost async output that would previously work just fine in 1.0.

My first idea for a fix would be to let cells unregister the handlers on clear output, etc., but this doesn't solve the issue for comms and widgets, which are the main reason for the patch. Perhaps a more generic notion of owners, and only allowing one outstanding msg_id per owner would allow the right behavior.