Add watchdog feature

[JustinAzoff]

The main loop may block for some external reason. dumbno should use signal.alarm with a few minute timeout inside the main loop. Nothing dumbno does should take longer than a few seconds, let alone a few minutes.

The best thing to do in the case of something freezing is to commit suicide and restart. There is no harm in doing so since dumbno itself is stateless.

[JustinAzoff]

We ran into this again the other day after doing an upgrade on the switch. The underlying problem is likely in the jsonrpc lib somewhere, but having a global watchdog timeout would work even better.