Allow TransactionEditor to move publishing work to the daemons

Summary:
Ref T6367. This is similar to D11329, but not quite as ambitious.

Allow Editors to implement supportsWorkers() and move their publishing work into a daemon. So far, only Paste supports this.

Most of the complexity here is saving and restoring state across the barrier between the web process and the worker process, but I think this is ~90% of it and then we'll pick up a couple of random things in applications.

I'm primarily trying to keep this as gradual as possible.

Test Plan:

• Published transactions with and without daemon support.
• Looked at mail, feed.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: fabe, epriestley

Maniphest Tasks: T6367

Differential Revision: https://secure.phabricator.com/D13107