net: Make addr relay mockable, add test

Summary:

As usual:

    Switch to std::chrono time to be type-safe and mockable
    Add basic test that relies on mocktime to add code coverage

Backport of core PR18454.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8479