[avalanche] Improve the peer manager verify() method

Summary:
Use identity comparison when checking the peers and proof pool consistency, and add comments for what the comparisons are for. Not only the proof ids must match, but they are expected to be the same underlying pointer.

Ref T1854.

Test Plan:

ninja all check-avalanche

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10603