Certified Users Metrics

A scheduler will call a worker every about 30 mins to:
- query Redis to get a list of Certified User Quiz Submission buckets
- for each bucket:
 + get a list of users
 + for each user:
    - call Synapse to get his/her passing record
    - if they pass, put the userId to a redis zset for certified users
    - if the passing record is not empty, get the list of responses
    - for each response:
        + if it is correct, put the question index to a redis zset for passed questions
        + else, put the question index to a redis zset for failed questions