Fixes, one api extension
API extension is "callback" method on Behavior. This method wraps a callback so it can be invoked on an offline
behavior. Works as an adapter for non-Observable APIs with callbacks.
Fixes:
- Cleaned up AdministratorCommissioningServer a bit and made callbacks run in proper context
- Do not drop the timeout promise in AdministratorCommissioning (we do this a lot with timers)
- Make NetworkBehavior.start() properly a no-op if already started
- Fixed Part validating child ID too early upon insertion
Also better documented expected behavior of Startable and Cancellable interfaces.
