Async IPC server support

On some application, server may run out of handlers when performing many blocking I/O operations during processing of each call (e.g. calling another service, etc.). A viable solution is increasing number of handlers.
But this faces the problem that large amount of threads will consume much memory (stack, etc.), and performance issues either.

After HADOOP-12909, work on asynchronization has been done on caller-side. This is a similar proposal on server-side.

Suggesting the ability to handle requests asynchronously.
For example, in such server, calls may return a Future object instead of immediate value. Then sends response to client in onSuccess or onFailed callbacks.