copycat internals doc: session registration/keep-alive failures

[kuujo]

@ongardie pointed out that the documentation is incorrect WRT how clients handle a failure to register a session. Copycat provides a mechanism to allow clients to determine how such a failure should be handled - ConnectionStrategy. Internals documentation needs to be updated to better describe how failures to register a session and keep a session alive are handled. Also, related to this are state changes in the client. Clients transition between CONNECTED, SUSPENDED, EXPIRED, and CLOSED states based on their ability to communicate with servers, and the causes and implications of those state changes (e.g. loss of linearizable semantics) are not currently documented.