Handle problem resets in such a way that the LoncapaProblem generates the new random seed, and the CapaModule saves that seed

Move xqueue handler to new module format, and continue to distinguish grades set by xqueue from those set by ajax in stats

Make system.xblock_model_data more explicit to fix the problem where error descriptors can't get any data because the schema and the data don't match.

Use the XBlock library as the base for XModule, so that we can incrementally rely on more and more of the XBlock api

Make sequences record whether they have been visited or not (done somewhat implicitly now, would be nice to be more explicit)