5.0.0

This is a significant rewrite of the how the underlying communication is implemented within the signaller. Notably:

• All communication is now done via a pull-stream interface, which significantly simplifies connectivity.
• There is no concept of WebSocket connectivity with a signaller now at all, but simply combine this with either rtc-switchboard-messenger or just messenger-ws to get this connectivity. This is what rtc-quickconnect will do when it is upgraded to use this version of the signaller.
• Update peer:filter event arguments to match other peer:* events (#19)
• Combined with some changes in rtc-switchboard@2.0.0 a signaller will no longer emit peer:leave messages when another peer connected to the signalling server leaves the room. This has been done to prevent reliance on what should be considered a temporary connection state.