Lift inline state transactions into core (in Diffusion)

Summary:
Ref T1460. Ref T6403. Replace Diffusion::INLINEDONE with Transactions::INLINESTATE and generalize things enough that we can lift it into core.

The next change will lift Differential's similar implementation into the core.

Also start implementing a fix for T6403, providing an alternate hook for optional builtin transactions.

Test Plan: Changed inline state in Diffusion.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6403, T1460

Differential Revision: https://secure.phabricator.com/D12129