Make THsHaServer.invoker available (get method only) in inherited classes

In some cases (for example in Apache Camel component for Thrift) there is a requirement, when it is necessary not only to transfer executorService from the external system through the Args in THsHaServer , but to organize control them from outside. In this case, it's possible to create a class which is inherited from THsHaServer, but not possible to access invoker in overloaded gracefullyShutdownInvokerPool(). As workaround the TNonblockingServer must be extended but requires to create several methods from scratch.
It's necessary to add code below to THsHaServer

protected ExecutorService getInvoker() {
  return invoker;
}

I'm ready to add this code as PR.