Resolved forgotten chain.h merge.
Summary: Continues work on T380, dependent on D1656. Fixes D1659. Removed CBlockStatus class from chain.h and placed into its own .h file, blockstatus.h. Updated other .h and .cpp files for dependencies.
Test Plan: make check
Reviewers: jasonbcox, deadalnix, schancel, O1 Bitcoin ABC, #bitcoin_abc
Reviewed By: jasonbcox, deadalnix, O1 Bitcoin ABC, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D1719