Fixed problems with psychometrics (Opaque Keys)

@carsongee Some simple fixes to get psychometrics working again:
- Update key comparisons for Opaque Keys
- Search for 'problem_check' instead of 'save_problem_check' when
  filtering TrackingLogs.
Just style, and I'm not sure it is any better than the above, but maybe:
```python
problems = dict(
    (
        p,
        pmdset.filter(
            studentmodule__module_state_key=BlockUsageLocator.from_string(p)
        ).count()
    ) for p in plist
)
```
makes this line a little easier to understand.