grpc-js: Remove watcher from queue before calling watcher callback. F…

…ixes grpc#1352

In the case where a new watcher is synchronously added to the watcher queue via the
watcher callback, this can result in the callback being called multiple times.

To support this case, the watcher needs to be move removed from the queue
before calling the watcher callback.

packages/grpc-js/src/channel.ts

@@ -381,9 +381,9 @@ export class ChannelImplementation implements Channel {
     const watchersCopy = this.connectivityStateWatchers.slice();
     for (const watcherObject of watchersCopy) {
       if (newState !== watcherObject.currentState) {
-        watcherObject.callback();
         clearTimeout(watcherObject.timer);
         this.removeConnectivityStateWatcher(watcherObject);
+        watcherObject.callback();
       }
     }
   }