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 bin/storage commands, ran bin/storage --host ....

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11044

Differential Revision: https://secure.phabricator.com/D16115