Prevent a race in Phabricator workers

Summary: See D133. Workers can also be subject to the same race, invert the
row relationship in the same way.

Test Plan: Launched repository master daemons and some taskmasters and used
the Daemon console to veify that they were able to process tasks. Manually
checked the database to make sure data got linked correctly and that new data
was inserted correctly.

Reviewers: jungejason

CC: tuomaspelkonen

Differential Revision: 135