scheduler: Replace stop(true) with StopWhenDrained()

Summary:
This helps understanding the code at the call site without having to
look up the name of the argument or the default value.

This is a backport of core#19090 [5/6]
https://github.com/bitcoin/bitcoin/pull/19090/commits/fac43f9889f500bcb62d830c030dec42fe791031

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9959