Fix visibility of the PhabricatorWorker::doWork() methods
Summary: Ref T6822. This method is only called from within the PhabricatorWorker::executeTask() and PhabricatorWorker::scheduleTask() methods.
Test Plan: grepped for ->doWork.
Reviewers: #blessed_reviewers, epriestley
Reviewed By: #blessed_reviewers, epriestley
Subscribers: Korvin, epriestley
Maniphest Tasks: T6822
Differential Revision: https://secure.phabricator.com/D11406