FirebaseEventTrackerFactory
Turns an event into the Firebase calls it should produce — or declines it.
Resolution.Declined passes the event to the next link in a CompositeFirebaseEventTrackerFactory; Resolution.Claimed and Resolution.Dropped both claim it and end the search, the second meaning "mine, and it goes nowhere".
A factory only answers for the events it knows, so a feature module can ship one for its own events and decline the rest.