Pass lifetimebound reference to SingleThreadedSchedulerClient

Summary:

Currently a pointer is passed, which is confusing and requires run-time asserts to avoid nullptr dereference.

All call sites can pass a reference, so do that. Also mark it LIFETIMEBOUND to avoid call sites passing a temporary. Also, unrelated cleanup in touched lines.

This is a backport of core#25040

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D13127