Separate CSeederNode class declaration from definition

Summary:
Separating the declaration from the definition of this class to allow
unit testing in the future without having to include bitcoin.cpp.

Test Plan:

make check

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

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4298