This is a first step toward replacing the orphan pool with the proof pool. This updates the PeerManager::addProofToPool method to handle more than a single proof per utxo in the pool. The decision of whether proof replacement should be allowed is moved to a bool argument to the method because it will be set in 2 cases: one for actually invalid proofs (the current orphans, which might become valid in the future), and one to work with the -enableavalancheproofreplacement flag being unset.
Ref T1854.
Depends on D10491.