[backport#16908] txmempool: Make entry time type-safe (std::chrono)

Summary:
https://github.com/bitcoin/bitcoin/pull/16908/commits/faec689bed7a5b66e2a7675853d10205b933cec8

Depends on D6524

Concludes backport of Core PR16908

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6527