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, what's the plan for this in production? We should really be generating the indexes once per server per index definition change and not having apps block on index creation which can take a long time. The other trouble w/ doing these inline is that if the index conflicts w/ an existing index (same keys but different sorts or optional parms), it will generate an error.