[Bug] Message to get topic is empty
As indicated in #19, the message sent to the `get` topic of each device is not valid.
It should contain all the keys we want to retrieve data for.

This means that each `ServiceWrapper` should somehow expose which keys it uses to determine the state, so we can include each of those in this "query" message.
I'm not yet sure how this will go into the interface (also see #18 as this will probably be taken into account there).