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/LocalStatus.py

@@ -161,6 +161,8 @@ class LocalStatusFolder(BaseFolder):
         self.readstatus(file)
         file.close()
 
+    def dropmessagelistcache(self):
+        self.messagelist = None
 
     def save(self):
         """Save changed data to disk. For this backend it is the same as saveall."""