static synchronized ExecutorService getExecutorService() {
      if (execService == null) {
        initializeExecutorService();
      }
      assert execService != null;
      return execService;
    }
Closes the executor service. This is called from
{@link PartitionedRegion#afterRegionsClosedByCacheClose(InternalCache)}
geode-core/src/main/java/org/apache/geode/internal/cache/PRQueryProcessor.java