[avalanche] Use the ProofPool multi index to store the orphan proofs

Summary:
This replaces the orphan pool, and reuse the same structure and wrappers to store the orphans proofs mapped by utxo and id. The only change in behavior is that we don't allow orphans with conflicting utxos anymore, which requires an update in a functional test and a new unit test case.

Ref T1854.

Depends on D10597.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10523