Features:
+* in pid1: before processing subscribed_as_strv when we come back from a
+ bus disconnect, let's check the bus 128bit id reported by the bus driver: if
+ it doesn't equal the one from before we should skip reinstalling the bus
+ tracker objects.
+
* account number of units currently in activating/active/deactivating state in
each slice, and expose this as a property of the slice, given this is a key
metric of the resource management entity that a slice is. (maybe add a 2nd