Define re-sync behavior for capabilities on network reconnection

[markafoltz]

If two agents are using the protocol to manage shared state, and there is a complete network disconnection followed by a reconnection, the spec should have something to say about what happens to that shared state.

The exact solution will depend on the capability/sub-protocol in use. For example:

• For a presentation connection, either side could queue and deliver messages, or we could close() events and have applications re-send failed messages.
• For remote playback, we could require a full sync of the media element state from the remote playback receiver to the controller.
• For streaming, we may require a PLI or other mechanism to synchronize codecs and media timelines.