Create indexes before running cross-modulestore import/export tests
Add an index on _id.revision to allow separating draft and published during export
Remove unneccesary uses of SON (which is slower than dict)
Only format test assertion messages when they are needed (they are a performance hit when testing large courses)
Merge pull request #5162 from cpennington/split/import-export-performance

Performance improvements to split mongo and old-mongo, based on import/export tests.