Prepare mail transmission to support failover across multiple mailers

Summary:
Ref T13053. Ref T12677. This restructures the calls and error handling logic so that we can pass in a list of multiple mailers and get retry logic.

This doesn't actually ever use multiple mailers yet, and shouldn't change any behavior. I'll add multiple-mailer coverage a little further in, since there's currently no way to effectively test which of several mailers ended up transmitting a message.

Test Plan:

• This has test coverage; tests still pass.
• Poked around locally doing things that send mail, saw mail appear to send. I'm not attached to a real mailer though so my confidence in local testing is only so-so.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13053, T12677

Differential Revision: https://secure.phabricator.com/D18998