Switch scheduler to steady_clock

Summary:
The scheduler should not be impacted by a system time change from the user nor NTP. This diff makes the scheduler use the steady clock which is monotonic.

This is a backport of core#25100.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12091