Remove peer:leave, replace with peer:heartbeat

[DamonOehlman]

I think one of the weaknesses in our current implementation is that while our primary use of websockets for signalling is good, the entanglement of peer states based on websocket connectivity vs RTCPeerConnection connectivity is bad.

As part of us moving to some more meaningful top level event names (see rtc-io/rtc-quickconnect#29) I think we should completely remove peer:leave event and concept. This should be replaced with a peer:heartbeat event which provides an indication that a peer has indicated activity by some means. This in turn should not even be part of the signalling logic, but part of another distinct package that deals with how heartbeats are communicated as a number of solutions are viable.

Regardless, we should not be using peer:leave to provide us any sense of connection between peers, and in fact the only reason it is still required in our stack is because firefox doesn't correctly monitor it's own peer connections connectivity states:

https://bugzilla.mozilla.org/show_bug.cgi?id=852665

Apparently although being reported in 2013, this issue still has a status of NEW :/

[DamonOehlman]

This has been implemented in via a8eaffa and also through an update to rtc-switchboard which no longer distributes /leave messages when a socket is disconnected. I'll open a separate issue for the heartbeat.