[avalanche] Make the peer hold a shared pointer to the proof

Summary:
Doing so will make it possible to share the proof ownership while being
safe if the peer is deleted in the meantime.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9607