rpc: Use steady_clock for getrpcinfo durations

Summary:
Currently it uses GetTimeMicros, which is the system time. Using steady time instead, makes the code type safe and avoids spurious offsets when the system time adjusts.

This is a backport of core#25456

The Tick helper is a dependency for future backports (starting with core#25717)

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15085