//   immediately ready and must run on some thread eventually.
// * New/modified-earlier timer: The current timer implementation (see time.go)
//   uses netpoll in a thread with no work available to wait for the soonest
//   timer. If there is no thread waiting, we want a new spinning thread to go
//   wait.
// * Idle-priority GC: The GC wakes a stopped idle thread to contribute to