Follow-up PR for #18533 -- another small step towards getting rid of the
confusing "command" terminology. Also see PR #18610 which tackled the
functional tests.

Backport of core PR18937.

Depends on D8460.

The change to process_messages.cpp is not ported as this PR is
deliberately ported out of order. The process_messages fuzzer was added
in D8376 and reverted because it caused a TSAN failure; this diff is a
dependency for the fix.