bt5/erp5_wendelin/DataBucketStream/getBucketByX: Free memory after loading

After loading the data from the ZODB, the 'getBucketBy...' methods
now free unused memory. Before this patch, it was the responsibility
of the user to deactivate no longer needed buckets, now this happens
automatically.

bt5/erp5_wendelin/DocumentTemplateItem/portal_components/document.erp5.DataBucketStream.py

@@ -199,7 +199,10 @@ class DataBucketStream(Document):
     """
       Get one bucket
     """
-    return self._tree[key].value
+    persistent_string = self._tree[key]
+    v = persistent_string.value
+    persistent_string._p_deactivate()  # free memory
+    return v
 
   def getBucketByIndex(self, index=None):
     """