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
I think we should put a comment on whichever of these is the master one saying that this method should not be called by prod code but only by command line and test code.