Adjust the condition variable sleep loop to work correctly when code
reached by its internal CHECK_FOR_INTERRUPTS() call interacts with
another condition variable.

There are no such cases currently, but a proposed patch would do this.

Discussion: https://postgr.es/m/CA+hUKGLdemy2gBm80kz20GTe6hNVwoErE8KwcJk6-U56oStjtg@mail.gmail.com