Client design makes it too easy to leak bluetooth notify callbacks on disconnect

[bdraco]

The current design of bluetooth_gatt_start_notify returns two callables:

• One that the caller must call when the device disconnects
• One that the caller must call to unsubscribe from a notification

We should only return one callable and handle cleaning up disconnecting the callback in _on_bluetooth_device_connection_response so callers can't get it wrong and leak a callback

[bdraco]

I think there is also a bug were we can miss a notify because we do not set up the listener before we subscribe so if the device replies with a notify right away we drop it

[bdraco]

I need to fix the bug first before refactoring since we need a patch release for that. The refactoring to return a single callable would be a breaking change which requires a major version bump