Filter IPv6 by ASN
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

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