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 messages sent to two different targets and one message which may have been split or re-sent. It also makes it easier to know what to bin/mail show-outbound --id <id> and where to find the message in the web UI for additional information.
The X-Phabricator-Send-Attempt is a unique value per attempt. If two mail messages are delivered with the same attempt value, the split is probably downstream from Phabricator. If they have different attempt values, the split is probably in Phabricator.
(In this case, the split was somewhere downstream from us, since sending mail with /usr/bin/mail also resulted in duplicates.)