Use thread-safe atomic in perfmon seeder

Summary:

Also switch to chrono based types.

Partial backport of core PR17270:
https://github.com/bitcoin/bitcoin/pull/17270/commits/64e1e022cedf6776c5dffd488ca2e766adca5dc3

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6208