Add time helpers for std::chrono::steady_clock, Remove unused GetTimeSeconds

Summary:

Add time helpers for std::chrono::steady_clock

random: Add FastRandomContext::rand_uniform_delay

Remove unused GetTimeSeconds

Seems confusing to have this helper when it is possible to get the system time in a type-safe way by simply calling std::chrono::system_clock::now (C++11).

This patch replaces GetTimeSeconds and removes it:

• in bitcoin-cli.cpp by system_clock
• in test/fuzz/fuzz.cpp by steady_clock

This is a backport of core#24921 and core#25102

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13988