[avalanche] Keep the peer set up to date as the chain tip is updated.

Summary:
As per title. This ensures that the set of proofs remains up to date.

Depends on D8534

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8535