R1:d321cc810aab

Freeze "maniphest.gettasktransactions" and make status/priority transactions more consistent

Summary:
Ref T13020. See PHI221.

Freeze legacy method `maniphest.gettasktransactions` in favor of modern method `transaction.search`.

Remove legacy "null on create" behavior from Maniphest status and priority transactions. This behavior is obsolete with EditEngine, and leads to inconsistent transaction sets in the transaction record.

The desired behavior is that transactions which don't do anything (e.g.