Apply ported version of the mail loss debugging and explicit deletions patch, which I've been developing with the help of some adventurous users. Thanks!

This tracks all deletions that happen through user actions and adds a check to the sync making sure that only things that were explicitely deleted are removed during sync. If unsure, the sync now re-downloads (duplicates) instead of removing mails, which should be safer. Also adds a lot of conditional debugging and refactors open/close to duplicate less code.

Will has a ported version of this for 3.x, which will go into pim+ shortly.