[avalanche] Make ProofPool's for each proof logic public

Summary:
forEachProof() can be used more generically. A follow up patch will sum immature
proof stakes in the getavalancheinfo RPC.

This patch demonstrates its usage by replacing the guts of countProofs()

Test Plan:

ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12086