Use read replica and cache for top scores

The leaderboard feature should be able to tolerate some latency in the search results.  This PR changes the `get_top_submissions()` API call to use a cache and the read-replica by default.  The change is backwards-compatible and will require no modifications to the leaderboard PR in edx-ora2.

@stephensanchez please review.
For kwargs in doc, is it worth mentioning what the default value is?