ExecutorService getSerialExecutor(InternalDistributedMember sender) {
      ExecutorService executor;
      Integer queueId = getQueueId(sender, true);
      if ((executor =
          serialQueuedExecutorMap.get(queueId)) != null) {
        return executor;
      }
      // If executor doesn't exists for this sender, create one.
      executor = createSerialExecutor(queueId);

      serialQueuedExecutorMap.put(queueId, executor);

      if (logger.isDebugEnabled()) {
        logger.debug(
            "Created Serial Queued Executor With queueId {}. Total number of live Serial Threads :{}",
            queueId, serialQueuedExecutorMap.size());
      }
      stats.incSerialPooledThread();
      return executor;
    }
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