Improve clarity of makeseeds and increase max ipv6 ASNs

Summary:
Backport of core#24818

This patch increases the number of ipv6 seeds from 2 to 10 per ASN and
increases MIN_BLOCKS to 730000.

Most of the refactors are not big improvements in clarity but it does bring
our code more inline with Bitcoin Core.

Test Plan:

python3 makeseeds.py < seeds_main.txt > nodes_main.txt
python3 makeseeds.py < seeds_test.txt > nodes_test.txt
git diff

Diff should be more additions than removals and retain most of the old entries.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12141