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/LocalStatusSQLite.py

@@ -203,6 +203,9 @@ class LocalStatusSQLiteFolder(BaseFolder):
             self.messagelist[uid]['labels'] = labels
             self.messagelist[uid]['mtime'] = row[2]
 
+    def dropmessagelistcache(self):
+        self.messagelist = None
+
     # Interface from LocalStatusFolder
     def save(self):
         pass