Wrap seeder options in a namespace

Summary: Prevent symbol collisions with seeder-specific constants. This is most relevant when writing tests.

Test Plan:

ninja check-seeder bitcoin-seeder

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10848