Add CONDSTORE support to sync

[djmaze]

The IMAP Extension for Conditional STORE Operation or Quick Flag Changes Resynchronization allows to quickly resynchronize mailbox flag changes. It seems to be supported on at least 75% of all IMAP servers. (That post is not even recent, so it probably is even more today.)

The sync feature should support this and use the slower traditional envelope sync as a fallback.

(The CONDSTORE feature could be useful for other usecases, but that should probably not be part of this issue.)