Provide a generic transaction-oriented bulk job worker

Summary:
Depends on D18806. Ref T13025. See PHI173. Currently, Maniphest bulk edits are processed by a Maniphest-specific worker. I want to replace this with a generic worker which can apply transactional edits to any object.

This implements a generic worker, although it has no callers yet. Future changes give it callers, and later remove the Maniphest-specific worker.

Test Plan: See next changes.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13025

Differential Revision: https://secure.phabricator.com/D18862