We updated scheduled admin checks to run concurrently in their own jobs. The main reason for this was so that we can implement re-check functionality for especially flaky checks (e.g. group e-mail credentials check.)
This works in the following way:
1. The check declares its retry policy using class methods.
2. A block can be yielded to if there are problems, but before they are committed to Redis.
3. The job uses this block to either a) schedule a retry if there are any remaining or b) do nothing and let the check commit.
This PR does some preparatory refactoring of scheduled admin checks in order for us to be able to do custom retry strategies for some of them.
Instead of running all checks in sequence inside a single, scheduled job, the scheduled job spawns one new job per check.
In order to be concurrency-safe, we need to change the existing Redis data structure from a string (of serialized JSON) to a list of strings (of serialized JSON).
