private void awaitTermination(long time, TimeUnit unit) throws InterruptedException {
      long remainingNanos = unit.toNanos(time);
      long start = System.nanoTime();
      for (ExecutorService executor : serialQueuedExecutorMap.values()) {
        executor.awaitTermination(remainingNanos, TimeUnit.NANOSECONDS);
        remainingNanos = (System.nanoTime() - start);
        if (remainingNanos <= 0) {
          return;
        }
      }
    }
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