Commit c6c9474a switched to condition variables instead of sleep
loops to notify backends of checkpoint start and stop, but forgot
to broadcast in case of checkpoint failure.

Author: Thomas Munro
Discussion: https://postgr.es/m/CA%2BhUKGJKbCd%2B_K%2BSEBsbHxVT60SG0ivWHHAdvL0bLTUt2xpA2w%40mail.gmail.com