Features/decay scores

[steffencruz]

Introduces a decay alpha which decreases ALL scores by a small amount of every step. This is effectively a ping for the miners so that those which are not alive pay a cost for not participating.

An initial value of decay_alpha=0.001 is set which would reduce all scores by 0.001 on every run_step. Given that currently step_size=50, the expected number of steps before a given UID is queried is around 1000/50 = 20.

This means that a UIDs score will tend to decrease by 0.02 between subsequent queries due to score decay, which is negligible compared to the ~0.3 that is typically scored. The net effect for active UIDs is a uniform drop in all scores by 0.02.

However, for UIDs that are not active this will compound so that their scores drop by 0.02 each time a query is missed - so after 10 missed queries (or 200 steps) their scores will drop by 0.2. In other words, miners that are not consistently alive will be at risk of deregistration.

• EMA alpha is increased from 0.05 -> 0.1.

TODO: unit tests for alpha decay

[p-ferreira]

I like how clean this solution is and how this change puts the expectation on reliability and consistency high, at the same time this makes harder for miners to perform any eventual non-instant update or adjustment.

I think it's worth highlighting this in the release so miners are aware that once they are registered, they will be actively punished for any downtime.