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/Maildir.py

@@ -220,6 +220,9 @@ class MaildirFolder(BaseFolder):
         if self.messagelist is None:
             self.messagelist = self._scanfolder()
 
+    def dropmessagelistcache(self):
+        self.messagelist = None
+
     # Interface from BaseFolder
     def getmessagelist(self):
         return self.messagelist