log: Use steady clock for bench logging
Summary:
Instead of using 0.001 and similar constants to "convert" an int64_t to milliseconds, use the type-safe Ticks<> helper. Also, use steady clock instead of system clock, since the durations are used for benchmarking.
Backport of core#26118.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D18873