[tools] add PoissonNextSend method that returns mockable time

Summary:

Introduce a Poisson helper method that wraps the existing method to return std::chrono::duration type, which is mockable.

This is a backport of Core PR17243

Test Plan: ninja && ninja check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8252