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
Mongo only cares about ordering when looking at document equality. In this case, each element of the dict is a separate clause in the query, and we're just ANDing those clauses together.