[avalanche] Protect nodes with high availability score from eviction

Summary:
This protects up to 128 nodes that participate to the avalanche network from being evicted.
This is complementary to the main protection mechanism (to be developped) that will protect the best node for each peer.

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/D9857