[avalanche] Make the proof comparator able to work with proofs and proof pointers

Summary:
As suggested by @deadalnix https://reviews.bitcoinabc.org/D10342#inline-48629, this makes the comparator more generic and is basically free.

Because the unit test uses shared pointers, there is no loss of test coverage.

Ref T1854.

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10355