Fix some caching issues in the CMS

[dandrabik]

WHAT

In the CMS, we have some queries that are taking a very long time and some caches that are never getting set because the payload is too large.

WHY

Generally, fast, cached things are good.

Also, multiple long-running queries at once that miss a cache and take longer than 5 seconds could cause the wave of ActiveRecord - could not obtain a connection from the pool within 5.000 seconds (waited 5.000 seconds); all pooled connections were in use that we saw earlier in the day. A class full of students starting the same activity at the same time would create the conditions to make this possible. Trying this fix before adjusting the connection pool size.

HOW

1. For responses_for_questions cache the ids instead of the results, since the results are too large for some questions
2. For the multiple_choice_options optimize the queries.
3. For both, expire the cache less frequently, and only delete it when necessary.

Screenshots

Memcache error

PR Checklist	Your Answer
Have you added and/or updated tests?	(No, should be the same business logic as before, so tests should cover.)
Have you deployed to Staging?	(Not yet, deploying now)
Self-Review: Have you done an initial self-review of the code below on Github?	Yes
Design Review: If applicable, have you compared the coded design to the mockups?	(N/A )

[cissyxyu]

Thanks for the comments!