Add a one-packet "stash" to QueuePacketConn.

Add a one-packet "stash" to QueuePacketConn.

I want a way to "unread" a packet from an send queue, in the case where
I'm packing packets into a fixed space and don't know when I'm done
until I've read one too many packets. There's no way to insert the extra
packet at the head of the cannel representing the send queue, so that it
will be the next thing received from OutgoingQueue. Instead, add an
separate one-element queue called the stash. The caller can stash an
excess packet, then prioritize checking it in the next round by calling
Unstash before OutgoingQueue.