Refactor communication APIs to support more efficient messaging

[kuujo]

The current API used by Copycat for messaging between the client and server precludes certain optimizations. In particular, events published from servers to clients require responses that are unnecessary under normal operation. Clients submit keep-alives that ensure events can be removed from memory on all servers, and clients need only send failure messages back to the server when an event arrives out of sequence.

Copycat 2.0 should implement a more high-level protocol API that contains Copycat-specific methods that allow for better optimization of communication between Copycat clients and servers.