Add test coverage to the PasswordEngine upgrade workflow and fix a few bugs

Summary:
Ref T13043. When we verify a password and a better hasher is available, we automatically upgrade the stored hash to the stronger hasher.

Add test coverage for this workflow and fix a few bugs and issues, mostly related to shuffling the old hasher name into the transaction.

This doesn't touch anything user-visible yet.

Test Plan: Ran unit tests.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13043

Differential Revision: https://secure.phabricator.com/D18897