Move PhabricatorModularTransaction slightly closer to having "final" methods again

Summary: Depends on D19290. Ref T13110. Differential still has some hacks in place which require these methods to "very temporarily" be nonfinal, but the badness can be slightly reduced nowadays.

Test Plan: Loaded some pages, nothing fataled.

Maniphest Tasks: T13110

Differential Revision: https://secure.phabricator.com/D19291