For FCM token inconsistencies, during development (upgrading app, un-re-install), it may result in bogus tokens sitting on the UPS for the device, leading to issues w/ duplicated messages or just incorrect tokens.

Also, the canonical IDs seems to be inconsistently applied to FCM's InstanceID tokens

So, we need to add a property to UPS's register method : oldToken
this is string (or array of strings?).

If not empty, the registration tiggers a service call which:

• looks up the installations matching the given token(s)
• removes them from the database

Let's not bake this into the Installation metadata, since it's not really belonging there...