R1:ffcfc0465226

Add some delivery diagnostic headers to outbound mail

Summary:
Fixes T13087. Ref T13090. An install ran into a situation where mail was being double-delivered, and it wasn't immediately clear where in the pipeline the issue lay.

This change adds some headers which should rule out (or, at least, render very unlikely) some possible causes if we encounter similar issues in the future.

The `X-Phabricator-Mail-ID` header stores the ID of the `MetaMTAMail` storage object so we can distinguish between two…