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/Base.py

@@ -247,6 +247,9 @@ class BaseFolder(object):
 
         raise NotImplementedError
 
+    def dropmessagelistcache(self):
+        raise NotImplementedException
+
     def getmessagelist(self):
         """Gets the current message list.
         You must call cachemessagelist() before calling this function!"""