Make "phd start" and "phd reload" use the process list, not PID files

Summary:
Ref T13321. This gets rid of the last pidfile readers in Phabricator; we just use the process list instead.

These commands always only work on the current instance since they don't make much sense otherwise.

Test Plan: Ran bin/phd start and bin/phd reload with and without daemons running.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13321

Differential Revision: https://secure.phabricator.com/D20606