Flows: model simple scoring function to prioritize running flows in the same time slot

[zaychenko-sergei]

Currently all flows are equal, and only the activation time and order of arrival define the order of tasks scheduling.

Prototype a smarter algorithm that provides more fairness:

• the activation time for flows should be rounded to the reasonable period (i.e. 1 minute), so that a minor difference like 1s won't be breaking the scoring functions' logic
• time wheel's priority queue could use a more complex key, that represents both rounded activation time + a priority score, so that flows are properly grouped by rounded activation time, but stay sorted by score within the same activation time
• implement demo of score computing logic:
  • rarely executed flow goes first (compare schedule periods)
  • flow with higher derived fanout goes first

In future, the scoring function will be more sophisticated, and will include preference of premium datasets or some sort of QoS guarantee (weight of paid datasets, delay penalty, fairness improvements, average work amount stats)