Implement bulk operations on the split modulestore.

The counts have nothing to do w/ this test but instead w/ the `setup_modulestore` fn (which we can use bulk in too, but it's not that toc is expensive in split); so, we should probably only measure it on one method not both of these.

Adding `with bulk` to `courseware.model_data.FieldDataCache.cache_for_descriptor_descendents` changed the counts to 4 & 9! Moving the `which check_mongo_calls` down one line (don't count `get_course`) reduces it to 1 & 5.