_This change should have no user impact._
This cleans up the way we generate registration IDs. It:
- Rewrites the generation code in Swift
- Removes the "sneaky transaction" method. I did this for clarity, and
because we won't use it for much longer
- Turns a log warning into a regular log—generating a registration ID is
not a problem
- Adds tests
I think this is a useful change on its own, but will be handy for an
upcoming change, too.
56589799cb