[backport#16908] util: Add count_seconds time helper

Summary:
https://github.com/bitcoin/bitcoin/pull/16908/commits/faaa1f01daba94b021ca77515266a16d27f0364e

Depends on D6523

Partial backport of Core PR16908

Test Plan:

ninja

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6524