We need to improve the comments explaining the main ideas / insights of the solution

Regressed in DMOJ/judge-server#673, since now the cache is per-worker-process, and gets destroyed on exit. Will require a rewrite of the caching system.

In #549, we added unittests for dispatch AtCoder and Codeforces.
To increasing test coverage, we should add unittests for other contest (HackerRank, CS Academy, ...).

Not only tests for all other contests, tests for the part of contests are welcome.

We should add more data to the judgehosts overview page:

• which judging is currently judged by each judgehost
• summary line with load averages