Apply Owners reviewers using ModularTransactions

Summary: Ref T10967. See that task for some discussion. This lets us do double writes on this pathway.

Test Plan: Set an Owners package to auto-review. Created revisions which triggered it: one with no reviewers (autoreview added); one with the package as a blocking reviewer explicitly (no automatic stuff happened, as expected).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10967

Differential Revision: https://secure.phabricator.com/D17512