Add daemon overseer modules to allow daemons to be externally reloaded
Summary: Ref T7053. Add a PhutilDaemonOverseerModule class which can be used to externally influence daemons.
Test Plan: See D14458.
Reviewers: #blessed_reviewers, epriestley
Reviewed By: #blessed_reviewers, epriestley
Subscribers: Korvin
Maniphest Tasks: T7053
Differential Revision: https://secure.phabricator.com/D14452