synchronized void closePool() {
    ExecutorService batchPool = this.batchPool;
    if (batchPool != null) {
      ConnectionUtils.shutdownPool(batchPool);
      this.batchPool = null;
    }
  }
The connection implementation based on {@link AsyncConnection}.
hbase-client/src/main/java/org/apache/hadoop/hbase/client/ConnectionOverAsyncConnection.java