R1:4f018488ae79

Restore Lisk transactional methods

Summary:
Restores a (simplified and improved) version of Lisk transactions.

This doesn't actually use transactions anywhere yet. DifferentialRevisionEditor
is the #1 (and only?) case where we have transaction problems right now, but
sticking save() inside a transaction unconditionally will leave us holding a
transaction open for like a million years while we run Herald rules, etc. I want
to do some refactoring there separately from this diff before making it…