R1:b469a5134ddd

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…