@Override
          public void run(PRESP resp) {
            if (controller.failed()) {
              future.completeExceptionally(controller.getFailed());
            } else {
              try {
                future.complete(respConverter.convert(controller, resp));
              } catch (IOException e) {
                future.completeExceptionally(e);
              }
            }
          }
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