[Part 1 of 5] Add a CChainState class to clarify internal interfaces

Summary:
Move txindex/undo data disk location stuff out of ConnectBlock

Backport of Core PR10279 commit 50701ba

Test Plan:

make VERBOSE=1 check && ./test/functional/test_runner.py

Reviewers: #bitcoin_abc, jasonbcox, deadalnix

Reviewed By: #bitcoin_abc, jasonbcox, deadalnix

Subscribers: deadalnix, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1968