DumpMempool: Use std::chrono instead of weird int64_t arthmetics

Summary:
This makes it so that DumpMempool doesn't depend on MICRO anymore

This is a partial backport of core#25487
https://github.com/bitcoin/bitcoin/pull/25487/commits/bd4407817e523e3c5b347bc6be25ed007cb27034

Depends on D15715

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15716