[avalanche] Add a rescan method to the ProofPool

Summary:
Similar to what is done in the OrphanProofPool, with a test.

Ref T1854.

Test Plan:

ninja all check-avalanche

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10598