R1:ecc598f18dc4

Support multiple database masters and convert easy callers

Summary:
Ref T11044. This moves toward partitioned application databases:

- You can define multiple masters.
- Convert all the easily-convertible code to become multi-master aware.

This doesn't convert most of `bin/storage` or "Config > Database (Stuff)" yet, as both are quite involved. They still work for now, but only operate on the first master instead of all masters.

Test Plan: Configured multiple masters, browsed around, ran…