Drop caches after having processed folders.

This enhances a lot memory consumption when you have many folders
to process.

Signed-off-by: Giovanni Mascellani <mascellani@poisson.phc.unipi.it>

offlineimap/folder/IMAP.py

@@ -250,6 +250,8 @@ class IMAPFolder(BaseFolder):
                 rtime = imaplibutil.Internaldate2epoch(messagestr)
                 self.messagelist[uid] = {'uid': uid, 'flags': flags, 'time': rtime}
 
+    def dropmessagelistcache(self):
+        self.messagelist = None
 
     # Interface from BaseFolder
     def getmessagelist(self):