OverflowQueueWithDMStats<Runnable> getSerialQueue(InternalDistributedMember sender) {
      Integer queueId = getQueueId(sender, false);
      if (queueId == null) {
        return null;
      }
      return serialQueuedMap.get(queueId);
    }
Holds info about unused thread, a thread is marked unused when the member associated with it
has left distribution system.
geode-core/src/main/java/org/apache/geode/distributed/internal/ClusterOperationExecutors.java