Implement task selection

[MattAlexMiracle]

This implements a simple self-stabilizing task selection system:

• ranking
• answering
• prompting
• voting

The goal of the algorithm task_selection is to balance prompting vs voting, such that the number of active prompts stays constant.
Further, we try to get tasks done as soon as possible, meaning that in the case a prompt is ready for ranking, we will preferentially (with probability p) rank first.

If the algorithm decides to "answer" a prompt, a second function next_answer_task exists that, given a dictionary of prompt-ids and existing number of answers, decides which prompt to provide an answer to next.
This also prefers close-to-done prompts so that we can start ranking quickly.

If the selected task doesn't exist (i.e. there are no prompts to answer) I would just re-sample which task to choose at the moment (though with the automatic balancing I'm not entirely sure whether this is even possible).

[andreaskoepf]

very nice, thank you!