HG-172: Decreased the service restart timeout

Make the service stop gracefully ASAP after `SIGTERM`.

Master now sends to all workers `shutdown` message on `SIGTERM`, when the workers receives the event, it calls `Hapi.Server.stop()`

Introduced new value to config `workerDisconnectTimeout` with default value 3000 ms.
It is used as timeout value for `Hapi.Server.stop` (which by default timeouts after 5000 ms) and in `server.js` for waiting the worker to disconnect.

Some non conclusive tests on laptop with 7 workers:

- 16ms from master receiving `SIGTERM` to last worker disconnected and shutdown, when no clients are connected;
- 93 ms from master receiving `SIGTERM` to last worker disconnected and shutdown, in the middle of ` ab -n 1000 -c 10 [url]`

The opaque value returned by setTimeout and setInterval also has the method timer.unref() which will allow you to create a timer that is active but if it is the only item left in the event loop won't keep the program running. If the timer is already unrefd calling unref again will have no effect.