[PATCH 4 of 4 stateful-chg] commandserver: allow service to process handler's heartbeat result

[Yuya Nishihara]

On Tue, 9 May 2017 10:37:25 -0700, Jun Wu wrote:
> Excerpts from Yuya Nishihara's message of 2017-05-09 22:57:23 +0900:
> > Does it mean we can't avoid sub-classing the service class?
> 
> If that's a goal, a possible solution is to move "_sock" state from service
> to handler.

The goal was to split application logic of chg from the service class
so that it would be reusable in prefork model (if any.) I have no idea
how and for what _handleheartbeat() will be used.