static void shutdownPool(ExecutorService pool) {
    pool.shutdown();
    try {
      if (!pool.awaitTermination(10, TimeUnit.SECONDS)) {
        pool.shutdownNow();
      }
    } catch (InterruptedException e) {
      pool.shutdownNow();
    }
  }
Select the priority for the rpc call.
<p/>
The rules are:
<ol>
<li>If user set a priority explicitly, then just use it.</li>
<li>For system table, use {@link HConstants#SYSTEMTABLE_QOS}.</li>
<li>For other tables, use {@link HConstants#NORMAL_QOS}.</li>
</ol>
@param priority  the priority set by user, can be {@link HConstants#PRIORITY_UNSET}.
@param tableName the table we operate on
hbase-client/src/main/java/org/apache/hadoop/hbase/client/ConnectionUtils.java