[avalanche] Check utxos integrity in the peer manager

Summary:
This extends the verify() method (testing only) to include a consistency check for the utxos map. This will be useful to make sure we are not making a mess when dealing with conflicting proofs.

Ref T1854.

Test Plan:

ninja check-avalanche

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10369