Clean up and expose filtered recipient lists for PhabricatorMetaMTAMail

Summary: Provide a public interface to get all the filtered recipients of an email. The intent is to pass this along to Notifications so it can mark notifications as read if the user is also receiving an email, possibly based on some preference (see T1403). This also simplifies the enormous sendNow() method a little bit.

Test Plan: Added unit tests, and sent a few mails that should cover most/all of these cases. They appeared to produce the correct recipients.

Reviewers: btrahan, vrana, nh

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1403

Differential Revision: https://secure.phabricator.com/D3778