refactoring: FlushStateToDisk -> CChainState

Summary:
Also renames global methods for clarity:

• ::FlushStateToDisk() -> CChainState::ForceFlushStateToDisk()
  • This performs an unconditional flush.

• ::PruneAndFlush() -> CChainState::PruneAndFlush()

This is a partial backport of Core PR15976 : https://github.com/bitcoin/bitcoin/pull/15976/commits/3ccbc376dd313fb7666471f24f6d9370914d00f3

Depends on D6001

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6002