Fix a threading issue with Amazon SES

Summary:
Amazon SES does not allow us to set a Message-ID header, which means
that threads are incorrect in Mail.app (and presumably other applications
which respect In-Reply-To and References) because the initial email does not
have anything which attaches it to the rest of the thread. To fix this, never
rely on Message-ID if the mailer doesn't support Message-ID.

(In the Amazon SES case, Amazon generates its own Message-ID which we can't
know ahead of time).

I additionally used all the Lisk isolation from the other tests to make this
testable and wrote tests for it.

I also moved the idea of a thread ID lower in the stack and out of
DifferentialMail, which should not be responsible for implementation details.

Test Plan:
Created and then updated a revision, messages delivered over Amazon
SES threaded correctly in Mail.app. Verified headers. Unit tests.

Reviewed By: rm
Reviewers: aran, tuomaspelkonen, jungejason, rm
Commenters: aran
CC: aran, rm, epriestley
Differential Revision: 195