node/chainstate: Decouple from GetTimeMillis

Summary:

...instead just move it out

Partial backport of core#23280:
https://github.com/bitcoin/bitcoin/pull/23280/commits/cbac28b72f5b831f6f84b7628f73e85627af3d94

Depends on D12555.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12556