Responder should shut down after connection terminates

[jonas-schievink]

Right now, when the link-layer connection is terminated, the packet queues might still contain data the responder will then still try to process. This might even "succeed" until the sending queue fills up. We should try to clear the queues and signal that the connection has shut down.

[jonas-schievink]

Looks like I thought about this in #26 already, but never implemented it:

For LL_TERMINATE_IND I think we'll want to quit the connection state and send the PDU through the channel, allowing the non-realtime part to later invoke a user callback.

This is not enough though, since the termination PDU will not be processed if the tx queue is full.