Compute graders performance optimization

@cahrens @dan-f @auraz 

After merging https://github.com/edx/edx-platform/pull/4425 rendering time of Justice course increased from 5.5 seconds to 8 seconds in my environment. This was caused by course graders being recomputed for each xblock. I propose we compute them just once.


why we need it here? it is never passed.