R1:1fc1114e2984

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…