WIP: Mirgate core sensors to use new sensor interface

Heh, I initially did exactly the same thing yesterday in my branch (used the `self._started` variable), but I didn't push it since I wanted to think some more about it.

@lakshmi-kannan And yeah, I was thinking we can maybe use `poll_interval` from metadata file to determine if the sensors is active (polling) or passive (non-polling).

I think something along the lines you have suggested is reasonable (just need to think more about it and see if we can come up with a better method name) :)