htlcswitch: add new LinkFailureDisconnect action

In this commit, we add a new LinkFailureDisconnect action that'll be
used if we detect that the remote party hasn't sent a revoke and ack
when it actually should.

Before this commit, we would log our action, tear down the link, but
then not actually force a connection recycle, as we assumed that if the
TCP connection was actually stale, then the read/write timeout would
expire.

In practice this doesn't always seem to be the case, so we make a strong
action here to actually force a disconnection in hopes that either side
will reconnect and keep the good times rollin' 🕺.

htlcswitch/linkfailure.go

@@ -64,6 +64,11 @@ const (
 	// LinkFailureForceClose indicates that the channel should be force
 	// closed.
 	LinkFailureForceClose
+
+	// LinkFailureDisconnect indicates that we should disconnect in an
+	// attempt to recycle the connection. This can be useful if we think a
+	// TCP connection or state machine is stalled.
+	LinkFailureDisconnect
 )
 
 // LinkFailureError encapsulates an error that will make us fail the current