subsequent resubscribers should not run if a resubscriber is successful

When investigating a case recently, I noticed many resubscribers running once a second. Looking at the code, it appears the kind of threadpool being used would just queue more and more up, but only run one at a time.

The subsequent resubscribers should exit immediately if resubscribe is no longer needed. Or the threadpool could be changed so we can't queue another while one is running.