[avalanche] Rename isValid => isBoundToPeer

Summary:
This better reflects what this does, i.e. checking if a peer is bound to that proof.

Ref T1854.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10557