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 actual change is largely just making sure that your config makes sense.

Test Plan:

• Configured db001.epriestley.com and db002.epriestley.com as master/master.
• Partitioned applications between them.
• Interacted with various applications, saw writes go to the correct host.
• Viewed "Database Servers" and saw partitioning information.
• Ran schema upgrades.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11044

Differential Revision: https://secure.phabricator.com/D16876