change builder2 so it NEVER attempts to cache Problem/TestCases

The Problem/TestCases can change in the webapp - for example,
if the instructor is editing an exercise.  So, for correctness,
the builder should always force the webapp to send the current
Problem/TestCases.
working on creating Problem/TestCases from Request

Pretty close: need to test and then work on handoff to builder
and support for waiting/polling for submission result.