[WIP] Introduce Firenest.ReplicatedState
@josevalim I think the local part is done. Now on to figure out the hard part - distributing the state.
I also haven't introduced the callbacks yet.
For anybody looking at the progress of this. We've changed the abstraction slightly into a ReplicatedState that should allow implementing something like phoenix presence, a process group or a rate limiter on top.
The code is not there yet, but the documentation is close to how I'd like to see it in the end.
Review is done, this looks fantastic! I really like the organization into three modules, it really helps keep things contained!