Pass callback through the worker supervisor state machine commands

[kriskowal]

e.g., In standby state, for the stop command, call callback on next tick.

https://github.com/uber/nanny/commit/5f44addd7f2a82cd5509d0dd09ef516a5b6fa878#commitcomment-8209862

[kriskowal]

This is a blanket ticket for addressing all concerns that @raynos mentions in linked commit feedback.