Enable updating task->revision assoc

Summary: add a conduit method to enable querying revisions' phid from
their revision_IDs, and another one to update the task->revision assoc.

Test Plan: for querying revision_phid method, tested empty, one, and two
revisions in the query. For the one to update the task->revision assoc,
I have another diff in facebook which verified it add and remove assoc
correctly.

Reviewers: tuomaspelkonen, epriestley

CC:

Differential Revision: 165