util: Allow use of C++14 chrono literals

Summary:

I think we should allow the use of chrono literals for new code to make it less verbose. Obviously old code can stay as-is.

This patch pulls in the needed namespace and replaces some lines for illustrative purposes.

Backport of core#20602.

Ref T1696.

Test Plan:

ninja all check-all

Build for Windows.

Reviewers: #bitcoin_abc, tyler-smith

Reviewed By: #bitcoin_abc, tyler-smith

Subscribers: tyler-smith

Maniphest Tasks: T1696

Differential Revision: https://reviews.bitcoinabc.org/D10901