[Part 3 of 5] Add a CChainState class to clarify internal interfaces

Summary:
Make DisconnectBlock unaware of where undo data resides on disk

Backport of core PR 10279 Commit 93a34cf.

Depends on D1969

Test Plan:

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

Reviewers: #bitcoin_abc, O1 Bitcoin ABC, jasonbcox

Reviewed By: #bitcoin_abc, O1 Bitcoin ABC, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1970