[seeder] Request headers from new connections

Summary:
The next step is to use the response to verify the connected node is (probably) on
the right chain using the latest checkpoint.

Depends on D6357

Test Plan:

ninja check-seeder

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, Fabien, nakihito

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6283