Update DNS seeds

Summary: As we'll mine bigger blocks, the network will partition. We need to rely on our own seeds.

Test Plan:

rm -rf ~/.bitcoin

And boostrap a bitocin abc node.

Reviewers: freetrader, sickpig, awemany, kyuupichan, Juan.Bitprim, #bitcoin_abc, hanchon

Reviewed By: freetrader, #bitcoin_abc, hanchon

Subscribers: hanchon

Differential Revision: https://reviews.bitcoinabc.org/D223