Improved signal handling for handlers

[jacobkiers]

The handlers within the manager do not have signal handling, a graceful kill can leave AlphaRPC to be in an incomplete state.

[jacobkiers]

Original comment from @reenl:

What we need:

• Register signal handlers
• Catch (zmq)poll exceptions
• Add stop, isRunning logic to handlers.

The catch is the polling is done by the alpharpc-*-handler scripts. If we add "run state" logic to the *Handler classes they will have even more responsibilities, which is undesirable.