Split/perf test

@nasthagiri @dianakhuang please review (@cpennington feel free to review at your discretion)

The first commit fixes a problem in split mongo where it wasn't properly serializing fields. The test in the last commit tests that code but I should probably add a test to ensure it properly deserializes. (I'll do that now).

The 4th commit (https://github.com/edx/edx-platform/commit/732940494584050e03796ee6381b2588bb90b040) fixes the test contextmanager.

commit (https://github.com/edx/edx-platform/commit/d5da3cf9f8dfd9cda78510654de04e4bd37a23c1) adds a hopefully useful test creation harness w/ a copy of toy created via modulestore commands rather than import and configurations for supporting split as well as mongo.
Why aren't the above lines implemented using the `check_exact_number_of_calls` contextmanager?