[avalanche] Improve const correctness of ProofPool::countProofs()

Summary:
ProofPool has historically been a construct only manipulated by PeerManager.
We can cleanup PeerManager's API by exposing the pools but first we need to
ensure ProofPool's API is also clean and acceptable for use outside of the
context of PeerManager. This means not accidentially altering pools that we
fetch from PeerManager.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12050