synchronize hot list periodically, also synchronize buffer list changes in partial sync mode

makes partial synchronization actually usable at last!

(problem: when in partial synchronization mode user opens and closes a buffer between synchronization events, the buffer might not hold the correct number of unread/hot messages, and the next update can erroneously show some meassages as unread/hot. not sure how deal with it. we only can update the whole buffer list when opening the channel. should we do this?)