Introduce a BlockStatus class to encapsulate CBlockIndex's validation status

Summary:
As per title.

Depends on D1445

Test Plan:

make check

Added a unit test for the new class.

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1450