R1:eb06aca951ce

Support DestructionEngine in MetaMTAMail

Summary:
Depends on D18984. Ref T13053. See D13408 for the original change and why this doesn't use DestructionEngine right now. The quick version is:

- It causes us to write a destruction log, which is slightly silly (we're deleting one thing and creating another).
- It's a little bit slower than not using DestructionEngine.

However, it gets us some stuff for free that's likely relevant now (e.g., Herald Transcript cleanup) and I'm planning to move…