Merge #10914: Add missing lock in CScheduler::AreThreadsServicingQueue()

Summary:
a56f8b0 Add missing lock in CScheduler::AreThreadsServicingQueue() (Matt Corallo)

Pull request description:

Not an actual bug as this is only used in asserts right now, but
nice to not have a missing lock.

Tree-SHA512: 7e542b150a0be716783e196493d239f2ad15e5376abf54b67d735dc3ef1b10849c090337b849f530c9f7497ddcfb8389b47d64a5dcf6382b7d38838f88cc1100

Backport of Core PR 10914
https://github.com/bitcoin/bitcoin/pull/10914/files

Test Plan: ninja check

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2943