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This makes reading the docs much easier as it's clear what's "private" and
what's not.

fixes #238

This patch tracks two usefulness metrics: short-term usefulness and long-term
usefulness. Short-term usefulness is sampled frequently and highly weights new
observations. Long-term usefulness is sampled less frequently and highly weights
on long-term trends.

In practice, we do this by keeping two EWMAs. If we see an interaction within
the sampling period, we record the score, otherwise, we record a 0. The
short-term one has a high alpha and is sampled every shortTerm period. The
long-term one has a low alpha and is sampled every longTermRatio*shortTerm
period.

To calculate the final score, we sum the short-term and long-term scores then
adjust it ±25% based on our debt ratio. Peers that have historically been more
useful to us than we are to them get the highest score.