[avalanche] Compute a node activity score

Summary:
This score reflects the node activity over the last invs polled. It ranges between 0 and 100 so it can be easily expressed as a percentage.
This score can be used for determining which node is the most reliable in a peer and protect it from eviction.

Ref T1634.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Maniphest Tasks: T1634

Differential Revision: https://reviews.bitcoinabc.org/D9839