Expose PeerConnectionState for monitoring

[poshcoe]

• Adds a new async state_change method to the PeerConnectionExt trait. This method can be awaited to receive WebRTC state changes and act accordingly (Closes Question: How to time out a connection? #3).
• Several trait methods now only require an immutable borrow thanks to use of flume across both native and web.
• Updated dependencies to latest versions, and removed patch version specifications.
• Updated signalling-peer example to use new state_change method to implement a single-threaded concurrent task for connection monitoring and recovery.