anounciness metric for peer selection

[coot]

List of changes

This PR introduces announcyness metric for peer selection which is used to
decide which hot peers we should demote.

For each peer we count how many valid signatures were announced before any
other peer announced it. This is implemented into two steps: first we account
at which time a peer announce a given signature id, then once we receive the
corresponding signature, and it turns out to be valid, we give a score of one
to the peer that announced it first. Each score is kept for an hour (the same
as churn rate).

Checklist

• related issue
• My changes generate no new warnings
• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works