Add mockable clock type and TicksSinceEpoch helper

Summary:
PR description:

This will be used primarily by the addr time refactor (core#24697) to make addr relay time type safe. However, it can also be used in other places, and can be reviewed independently.

This is a backport of core#25101
Depends on D14911

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14912