Make it possible to request stop of message loop

Add a `RequestStopMessageLoop()` method to `Ricochet` to be able to
stop handling new connections. Right now, ProcessMessages is an infinite
loop.

ricochet.go

@@ -93,10 +93,19 @@ func (r *Ricochet) processNewConnection(conn net.Conn, service RicochetService)
 func (r *Ricochet) ProcessMessages(service RicochetService) {
 	for {
 		oc := <-r.newconns
+		if oc == nil {
+			return
+		}
 		go r.processConnection(oc, service)
 	}
 }
 
+// Request that the ProcessMessages loop is stopped after handling all currently
+// queued new connections.
+func (r *Ricochet) RequestStopMessageLoop() {
+	r.newconns <- nil
+}
+
 // ProcessConnection starts a blocking process loop which continually waits for
 // new messages to arrive from the connection and uses the given RicochetService
 // to process them.