Cache ChallengeType and ChallengeTrack lookups

[rakibansary]

ChallengeType and ChallengeTrack are fetched from DynamoDB with a scan operation for each /challenges call. These should be indexed in ES, but there's currently no processor for it. Since actual data is small enough, they can be cached in-memory.

[sandhiyakavi]

Sanity testing is completed in Prod Env. No issues found.