[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