Scheduler should handle signals for graceful shutdown

[icook]

In some rare occasions restarting will terminate a task in a critical area causing data loss. I believe that this isn't possible for paying out blocks, but it has definitely happened for one minute share processing.

[icook]

This might already be handled better in APScheduler 3.0, so that should be checked before rolling our own mess.

[ericecook]

This is available in 2.1.2 http://apscheduler.readthedocs.org/en/v2.1.2/modules/scheduler.html?highlight=shutdown#apscheduler.scheduler.Scheduler.shutdown

Should be pretty easy to implement