Support for Sessions

[drub0y]

Need to come up with a configuration method for indicating that sessions should be used to receive messages. Basic implementation would simply accept each session and iterate through all of its available messages before moving on to the next session. While this would work, it does not necessarily provide all the possible benefits of having sessions.

Should consider whether the fact that sessions are being used should be surfaced in the API by switching the result type to something like IObservable<IObservable<TMessage>> where each of the values emitted is an IObservable representing the messages within a session. Doing so would allow application logic to partition its work in terms of concurrency amongst sessions, but I'm not sure if the core Obvs [Configuration] API lends itself to presenting this kind of representation.