Move CBlockIndexWorkComparator to its own header

Summary: CBlockIndexWorkComparator belongs in its own header rather than validation.cpp. Also, this makes it more accessible for writing tests.

Test Plan: make check

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1536