[avalanche] Move the ProofPool definition to its own header
It will next be turned into a class so we can add methods like addProof and removeProof. There is no change in behavior.
ninja all check-avalanche
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Maniphest Tasks: T1854
Differential Revision: https://reviews.bitcoinabc.org/D10582