refactor: Replace GetTimeMicros by SystemClock
Summary:
It is unclear from the name that GetTimeMicros returns the system time. Also, it is not using the type-safe std::chrono types. Fix both issues by replacing it with SystemClock in the only place it is used. This refactor should not change behavior.
Backport of core#27233.
Depends on D18873.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D18874