fix: Add missing monitor from startSubscriber. (#142)

Author: dpcollins-google

google-cloud-pubsublite/src/main/java/com/google/cloud/pubsublite/cloudpubsub/internal/AssigningSubscriber.java

@@ -107,7 +107,9 @@ public void failed(State from, Throwable failure) {
 
  @Override
  public void terminated(State from) {
- stoppingSubscribers.remove(subscriber);
+ try (CloseableMonitor.Hold h = monitor.enter()) {
+ stoppingSubscribers.remove(subscriber);
+ }
  }
  },
  MoreExecutors.directExecutor());