Verify avalanche proof at init

Summary:
There are a few checks that can be performed immediately without
verifying all the utxos, and can provide immediate feedback to the user
if an ill-formed proof is supplied through -avaproof.

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9342