code: private synchronized void removeEmptyClaimSets() {
          Iterables.removeIf(_map.values(), new Predicate<Handle>() {
              @Override
              public boolean apply(Handle handle) {
                  handle.getClaimSet().pump();
                  return handle.getRefCount().get() == 0 && handle.getClaimSet().size() == 0;
              }
          });
      }
docs: Cleans up old claim sets for subscriptions that have become inactive.  Ensures the map of claim sets doesn't
  grow forever.