[avalanche] Track registered and allocated scores in PeerManager.

Summary:
When adding/remove peers/nodes we track the scores so we can quickly query the state for quorum
detection.

Test Plan: ninja check-avalanche

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Maniphest Tasks: T1864

Differential Revision: https://reviews.bitcoinabc.org/D11181