peer: ensure queueMsg won't create deadlock if peer shutting down

This commit adds some additional measures to ensure that a call to
queueMsg while the peer is shutting down won’t result in a potential
deadlock.

Currently, during shutdown the outgoingQueue channel is attempted to be
cleared by he writeHandler, however adding an additional select
statement serves as a mother layer of defense from nasty dead locks.

peer.go

@@ -607,7 +607,11 @@ out:
 // queueMsg queues a new lnwire.Message to be eventually sent out on the
 // wire.
 func (p *peer) queueMsg(msg lnwire.Message, doneChan chan struct{}) {
-	p.outgoingQueue <- outgoinMsg{msg, doneChan}
+	select {
+	case p.outgoingQueue <- outgoinMsg{msg, doneChan}:
+	case <-p.quit:
+		return
+	}
 }
 
 // ChannelSnapshots returns a slice of channel snapshots detailing all