Rename CMessageHeader in the seeder into CSeederMessageHeader

Summary: This ensure that we don't have name clashes between the seeder and the rest of the codebase and we can leverage this to improve code resuse.

Test Plan: Build the seeder.

Reviewers: schancel, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D726