Start all daemons under a single overseer

Summary:
Ref T7352. This moves all the daemons under one overseer. The primary goal is to reduce the minimum footprint of an instance in the Phacility cluster, by reducing the number of processes each instance needs to run on daemon-tier hosts.

This improves scalability by roughly a factor of 2.

Test Plan:

• Ran phd debug, phd launch, phd start`. Saw normal behavior, with only one total overseer.
• Fataled dameons and saw the overseer restar them normally.
• Used phd status and phd stop and got reasonable results (phd status is still a touch off).

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7352

Differential Revision: https://secure.phabricator.com/D11857