validation: use std::chrono in CChainState::FlushStateToDisk()
validation: use std::chrono in CChainState::FlushStateToDisk()

Summary:
This is a backport of [[https://github.com/bitcoin/bitcoin/pull/19088 | core#19088]]

The second commit related to fee estimation is not applicable.

Test Plan: `ninja all check-all`

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9268

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