[refactor] factor CBlockIndex out of chain.h

Summary: seems like the right thing to do. looks like we're just two small backports away from Core where CBlockIndex is concerned, so give it its own translation unit

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6988