misc rexpro fixes

A few updates based on my tests with rexpro-python.

* Session requests with null session keys return an appropriate error
* Integration tests touching the graphs will now loop over the available graphs and rerun the test body for each of them
* Finally, script response messages are serialized into bytes before transactions are closed.  This is kind of a weird one. Titan vertices were causing problems if the RexProMessageFilter tried to serialize them into byte arrays after their session had been closed. Auto-commit now works with titan, but we're now duplicating the serialization code, so this should probably be refactored into something a little cleaner in the future.