Bug 800402 - [email] UI: Show the number of unread messages in the message-list header after the folder name

let's lose the comment-block above.  For {start,end}{TS,UID} we realy only need one of the pairs to serve as a sort-of pointer.  Similarly we can get rid of the logic that handles both possible "dir"s.  I think it's preferable to start with the most recent and move "before"wards which corresponds to dir === 1.

This means we only need to track startTS/startUID.