Update Bitcoin Unlimited seeder

Summary:
Previuos seeder provides non BCC spec compliant peers.
The new one filters peers based on the new bit service
NODE_BITCOIN_CASH.

Test Plan: dig btccash-seeder.bitcoinunlimited.info

Reviewers: freetrader, deadalnix, awemany, #bitcoin_abc

Reviewed By: freetrader, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D376