chain: resolve data race in sequential start stop test

Resolve the data race in the sequential start stop unit test
by ensuring that each goroutine launched in the Start method
is properly accounted for in the client's waitgroup.

Add a TODO to shutdown the rescan goroutine when the Stop
method is called.

chain/neutrino.go

@@ -100,24 +100,36 @@ func (s *NeutrinoClient) Start() error {
 	s.clientMtx.Lock()
 	defer s.clientMtx.Unlock()
 	if !s.started {
+		// Reset the client state.
 		s.enqueueNotification = make(chan interface{})
 		s.dequeueNotification = make(chan interface{})
 		s.currentBlock = make(chan *waddrmgr.BlockStamp)
 		s.quit = make(chan struct{})
 		s.started = true
+
+		// Go place a ClientConnected notification onto the queue.
 		s.wg.Add(1)
 		go func() {
+			defer s.wg.Done()
+
 			select {
 			case s.enqueueNotification <- ClientConnected{}:
 			case <-s.quit:
 			}
 		}()
+
+		// Go launch the notification handler.
+		s.wg.Add(1)
 		go s.notificationHandler()
 	}
 	return nil
 }
 
 // Stop replicates the RPC client's Stop method.
+//
+// TODO(mstreet3): The Stop method does not cancel the long-running rescan
+// goroutine.  This is a memory leak.  Stop should shutdown the rescan goroutine
+// and reset the scanning state of the NeutrinoClient to false.
 func (s *NeutrinoClient) Stop() {
 	s.clientMtx.Lock()
 	defer s.clientMtx.Unlock()