pubsub: fix potential deadlock in startConnections (#1674)

If a subscriber connection fails, awaitRunning will throw
IllegalStateException.
We must make sure that we count down the latch in either case;
otherwise startup will deadlock.

Also move the call to addListener above startAsync.
Otherwise, the listener might not see the connection failing
if it fails too quickly.

google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/spi/v1/Subscriber.java

@@ -189,7 +189,7 @@ public boolean isRunning() {
    *   }
    * }, executor);
    * subscriber.startAsync();
-   * 
+   *
    * // Wait for a stop signal.
    * done.get();
    * subscriber.stopAsync().awaitTerminated();
@@ -466,9 +466,12 @@ private void startConnections(
             new Runnable() {
               @Override
               public void run() {
-                subscriber.startAsync().awaitRunning();
-                subscribersStarting.countDown();
                 subscriber.addListener(connectionsListener, executor);
+                try {
+                  subscriber.startAsync().awaitRunning();
+                } finally {
+                  subscribersStarting.countDown();
+                }
               }
             });
       }