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

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1719