[avalanche] Turn ProofRef into a RCUPtr

Summary:
As per title. This requires to replace the make_shared calls with a call to the RCU factory instead. A move constructor for the proof is needed in order to facilitate deserializing a proof then building the pointer.

Depends on D11509.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D11466