[avalanche] Move the ProofPool definition to its own header

Summary:
It will next be turned into a class so we can add methods like addProof and removeProof. There is no change in behavior.

Ref T1854.

Test Plan:

ninja all check-avalanche

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10582