refactoring: IsInitialBlockDownload -> CChainState

Summary:
We introduce CChainState.m_cached_finished_ibd because the static state it
replaces would've been shared across all CChainState instances.

This is a partial backport of Core PR15976 : https://github.com/bitcoin/bitcoin/pull/15976/commits/403e677c9ebbf9744733010e6b0c2d1b182ee850

Depends on D6002

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6035