Drop caches after having processed folders.

OfflineIMAP retains forever messagelist caches after having processed a folder. This means that memory consumption grows linearly with the number of processed folders. With this commit memory is reclaimed after having finished to process a folder. In my case (about 30 folders) the enhancement is significant, especially when running on lower-end machines.

I didn't perform particularly careful testing for this commit; I just saw it working in my case.