* Stub for polling from a central location
* Allow connection to know the entity objects attached to it
* Move polling logic to connection
* Don't poll if no characteristics selected
* Loosen coupling between entity and HKDevice
* Disable track_time_interval when removing entry
* Revert self.entities changes
* Use @callback for async_state_changed
* Split out unload and remove and add a test
* Test that entity is gone and fix docstring