Flip the "STARTTLS required" bit in settings when it suceeds for the first time

Thanks to Mike Cardwell on the trojita ML [1] for the initial suggestion.

[1] http://article.gmane.org/gmane.mail.trojita.general/299
Composer: save the draft right before we attempt sending it
Threading: fix a race condition which led to hiding certain messages

There was a problem when a message disappeared (EXPUNGE/VANISHED) between the
untagged and tagged THREAD responses. In these situations, all children of the
removed messages were temporarily hidden.