Separate CSeederNode class declaration from definition
Separating the declaration from the definition of this class to allow
unit testing in the future without having to include bitcoin.cpp.
Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc
Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D4298