In this commit, we add a new abstract message router. Over time, the
goal is that this message router replaces the logic we currently have in
the readHandler (the giant switch for each message).
With this new abstraction, can reduce the responsibilities of the
readHandler to *just* reading messages off the wire and handing them off
to the msg router. The readHandler no longer needs to know *where* the
messages should go, or how they should be dispatched.
This will be used in tandem with the new `protofsm` module in an
upcoming PR implementing the new rbf-coop close.
