Updated Duckietown-challenges server fixes speed problems; updated “dts commands evaluate”

As we have more participants, the Duckietown Challenges Server started to feel slow. The reason: we were a bit lazy and some pages had O(n) implementations where O(1) was needed - loading all challenges/submissions/etc.

We also updated the "dts challenges evaluate" command to be more robust. Please continue to report bugs as this part is fragile by nature --- running containers that spawn other containers on the user's machines.