[Part 5 of 5] Add a CChainState class to clarify internal interfaces

Summary:
Move some additional variables into CChainState private

Backport of Core PR 10279 commit 2862aca
https://github.com/bitcoin/bitcoin/pull/10279/commits/2862aca40f24acc48d3052d5ba3f639e3fc0a621

Depends on D1971 and D2645
T572

Test Plan:

make check
test_runner.py --extended

Reviewers: #bitcoin_abc, schancel, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: markblundeberg, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1972