[avalanche] Make the peermanager methods use a shared pointer to proof

Summary:
The public interfaces from the processor are not migrated yet and will
be in their own diff so the changes remain easy to review.

Depends on D9607.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9616