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

Builds on #5131.

Diff: https://github.com/cpennington/edx-platform/compare/cpennington:split/prod-import-export-tests...split/import-export-performance
So why doesn't it matter that the ordered dict of the SON is being removed here? Is there some other ordering mechanism at play? Does Mongo not care about the order?