The declared variable type is consistent with the enum type defined

Summary:
nStatus of CBlockIndex is consistent with the definition of Enum(BlockStatus)
The BlockHeader is consistent with the type of variable defined in CBlockHeader

Test Plan: make check

Reviewers: #bitcoin_abc!, deadalnix, freetrader, CCulianu, hanchon, O1 Bitcoin ABC!

Differential Revision: https://reviews.bitcoinabc.org/D528