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

Summary:
Similar to what is done for the peermanager, the processor can benefit
for dealing with proof shared pointers.

Depends on D9616.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9617