Extract out the next inventory send time computation

Summary:
This will be used for sending the proof invs as well. There is no change
in behavior.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1611

Differential Revision: https://reviews.bitcoinabc.org/D9654