R1:c45876841585

Fix various threading issues, particularly in Gmail

Summary:
- Add an explicit multiplexing option, and enable it by default. This is necessary for Mail.app to coexist with other clients ("Re:" breaks outlook at the very least, and generally sucks in the common case), and allows users with flexible clients to enable subject variance.
- Add an option for subject line variance. Default to not varying the subject, so mail no longer says [Committed], [Closed], etc. This is so the defaults thread correctly…