[avalanche] Move the utxo limitation to the proof verification

Summary:
This makes it possible to build a proof with too many utxos, but clean
the network layer from this verification.

Test Plan:

./test/functional/test_runner.py abc_rpc_avalancheproof

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9352