node/chainstate: Decouple from GetTime

Summary:

...instead pass in a std::function<int64_t()>
Note that the static_cast is needed (apparently) for the compiler to know which overloaded GetTime to choose.

Partial backport of core#23280:
https://github.com/bitcoin/bitcoin/pull/23280/commits/05441c2dc5f60e2025476d8ec94c9025032d118c

Depends on D12576.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D12577