Allow "SMTP" and "Sendmail" mailers to have "Message-ID" behavior configured in "cluster.mailers"

Summary:
Fixes T13265. See that task for discussion. Briefly:

• For mailers that use other mailers (SMTP, Sendmail), optionally let administrators set "message-id": false to improve threading behavior if their local Postfix is ultimately sending through SES or some other mailer which will replace the "Message-ID" header.

Also:

• Postmark is currently marked as supporting "Message-ID", but it does not actually support "Message-ID" on secure.phabricator.com (mail arrives with a non-Phabricator message ID). I suspect this was just an oversight in building or refactoring the adapter; correct it.
• Remove the "encoding" parameter from "sendmail". It think this was just missed in the cleanup a couple months ago; it is no longer used or documented.

Test Plan: Added and ran unit tests. (These feel like overkill, but this is super hard to test on real code.) See T13265 for evidence that this overall approach improves behavior.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13265

Differential Revision: https://secure.phabricator.com/D20285