CMFActivity: force using READ-COMMITTED isolation.

product/CMFActivity/ActivityTool.py

@@ -703,7 +703,9 @@ class ActivityTool (BaseTool):
         return meta_types
 
     def getSQLConnection(self):
-      return self.aq_inner.aq_parent.cmf_activity_sql_connection()
+      db = self.aq_inner.aq_parent.cmf_activity_sql_connection()
+      db._isolation_level = 'READ-COMMITTED'
+      return db
 
     def maybeMigrateConnectionClass(self):
       connection_id = 'cmf_activity_sql_connection'