[avalanche] Make the processor methods use a shared pointer to proof
Similar to what is done for the peermanager, the processor can benefit
for dealing with proof shared pointers.
Depends on D9616.
ninja all check-all
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D9617