R1:55e21565b593

Support application partitioning across multiple masters

Summary:
Ref T11044. I'm going to hold this until after the release cut, but I think it's good to go.

This allows installs to configure multiple masters in `cluster.databases` and partition applications across them (for example, put Maniphest on a dedicated database).

When we make a Maniphest connection we go look up which master we should be hitting first, then connect to it.

This has at least approximately been planned for many years, so the…