Summary: This adds methods to start and stop tracking any arbitrary PHID in phrequent. Currently, this uses copy-pasted code from PhrequentTrackController. I had to do this because the code to start/stop was not abstracted into a common class. Once the code to start/stop working is extracted into a re-usable class, the conduit API can use this as well. Test Plan: I called the functions with a PHID of a task and ensured that the fields in the phrequent database table was being updated correctly. Reviewers: skyronic, #blessed_reviewers, epriestley Reviewed By: #blessed_reviewers, epriestley Subscribers: maxhodak, erik.fercak, aran, epriestley, Korvin Maniphest Tasks: T3569, T3970 Differential Revision: https://secure.phabricator.com/D7326