Filter IPv6 by ASN

Summary:
Backport of Core PR15840
https://github.com/bitcoin/bitcoin/pull/15840/files

Test Plan: Run makeseeds.py according to contrib/seeds/README.md

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4779