Add a way to stop scheduleEvery instances.
Once a task is scheduled to happen on a regular basis, there are no ways to stop it. This diff fixes it.
It does so by making the scheduled function return a bool, which will determine if it is to be executed again.
Test Plan: Add a test case.
Reviewers: #bitcoin_abc, Fabien
Reviewed By: Fabien
Differential Revision: https://reviews.bitcoinabc.org/D2011