Increase performance of transforms

[bmtcril]

We profiled this in #289 and found that a huge amount of transform time is being taken up querying for user ids and course names, which we should be able to cache or find existing caches for.

We should also investigate fanning out multiple processes to ingest multiple files at a time and an option to create lookup tables of learner and course data that the process can work on outside of an LMS worker for extreme cases.