Conduit APIs to start and stop tracking time in phrequent

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