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