Synchronize folders properly

:pushpin: With this we won't fetch every email inside the folder but only the needed ones. Now we will (1) update change number of the folder with new contents and (2) store relationship between that change number and the current modseq (which will be use for future imap queries). :dancers:

:warning: This need to have https://github.com/openchange/openchange/pull/125 merged, otherwise bad things will happen :bomb: