Improve the ReattempInitalBroadcast interval computation

Summary: Use a better name and std::chrono literals to make it easier to read. There is no change in behavior.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11033