In this commit, we fully integrate the new RBF close state machine into
the peer.
For the restart case after shutdown, we can short circuit the existing
logic as the new FSM will handle retransmitting the shutdown message
itself, and doesn't need to delegate that duty to the link.
Unlike the existing state machine, we're able to restart the flow to
sign a coop close with a new higher fee rate. In this case, we can now
send multiple updates to the RPC caller, one for each newly singed coop
close transaction.
To implement the async flush case, we'll launch a new goroutine to wait
until the state machine reaches the `ChannelFlushing` state, then we'll
register the hook. We don't do this at start up, as otherwise the
channel may _already_ be flushed, triggering an invalid state
transition.
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.
