Backport FlushStateMode enum class

Summary:
Partial backport of Core PR 10742 (FlushStateMode only)
https://github.com/bitcoin/bitcoin/pull/10742/files
Progress towards T549

Test Plan: ninja check

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2716