I'm planning to use _one later, but it may be doubly registered by
switching `stop_worker_p` now and so we should not use _one for now.

Otherwise stale job may reject new job registration and
copy_cache_from_main_thread may wait forever.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65571 b2dd03c8-39d4-4d8f-98ff-823fe69b080e