[avalanche] pass chainman to Proof::verify

Summary:
This removes usages of the global chainstate in the Proof class.

We now require Proof::verify callers to hold the lock for chainmanager operations.

Depends on D11703

Test Plan:
With debug and clang:
ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D11704