@Override
  public boolean dispatch(CallRunner callTask) {
    // FastPathHandlers don't check queue limits, so if we're completely shut down
    // we have to prevent ourselves from using the handler in the first place
    if (currentQueueLimit == 0) {
      return false;
    }
    FastPathRpcHandler handler = popReadyHandler();
    return handler != null ? handler.loadCallRunner(callTask) : super.dispatch(callTask);
  }
Balanced queue executor with a fastpath. Because this is FIFO, it has no respect for ordering so
a fast path skipping the queuing of Calls if an Handler is available, is possible. Just pass the
Call direct to waiting Handler thread. Try to keep the hot Handlers bubbling rather than let them
go cold and lose context. Idea taken from Apace Kudu (incubating). See
https://gerrit.cloudera.org/#/c/2938/7/src/kudu/rpc/service_queue.h
hbase-server/src/main/java/org/apache/hadoop/hbase/ipc/FastPathBalancedQueueRpcExecutor.java