Correct "msort()" vs "msortv()" to more fully stabilize transaction sorts after recent changes

Summary: Ref T13303. I upgraded this to a vector-based sort but forgot to type a "v", which means the sort has different stability under PHP 5.5. See D20582 for a root cause fix.

Test Plan: Locally, on PHP7, not much changes. I expect this to fix the odd selection of title stories in mail and notification stories on secure, which is running PHP 5.5.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13303

Differential Revision: https://secure.phabricator.com/D20583