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 an implementation of a new interface the rbf coop
state machine needs. We take care to accept interfaces everywhere, to
make this easier to test, and decouple from the concrete types we'll end
up using elsewhere.
