Move seeder specific protocol parts in their own file.

Summary:
This will help to start using the existing libraries instead of the bad and unmaintained versions that exist for the seeder.

Depends on D718

Test Plan:

make check

Run the seeder.

Reviewers: schancel, sickpig, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D719