Move mapBlockIndex to chain.h

Summary: And use the opportunity to cleanup checkpoints.cpp in the process.

Test Plan:

make check
../qa/pull-tester/rpc-tests.py

Reviewers: schancel, Mengerian, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Subscribers: dagurval

Differential Revision: https://reviews.bitcoinabc.org/D712