Cleanup Session after each request.

It's good practice to cleanup the SQL session after each
request so that the next request gets a fresh one.

It's an application decision whether one wants a
just-in-case ROLLBACK or COMMIT. There are two ideas behind
it, really. I have decided for ROLLBACK. The idea is "if
you forget to commit your changes yourself, there's
something broken. Maybe you got an exception?".

mediagoblin/db/sql/open.py

@@ -36,6 +36,7 @@ class DatabaseMaster(object):
         Session.flush()
 
     def reset_after_request(self):
+        Session.rollback()
         Session.remove()