There is a common trick for reduction of contention on sync primitives. Unfortunately it
does not have a common name, but see mutexWaiterShift handling in sync/mutex.go for an
example. We can use it for async channels to reduce contention as well. Namely:

- add a waiterAwake flag to Hchan
- when a goroutine sends to async chan and there is a parked receiver, if
waiterAwake=false unpark it and set waiterAwake, otherwise do nothing
- if the woken receiver discovers empty queue, it resets waiterAwake before parking again
- if the woken receiver successfully dequeues from the queue, it does:
    if queue empty || no parked receivers {
      waiterAwake = false
    } else {
      unpark one parked receiver
      // responsibility to manage waiterAwake and subsequent parked receivers
      // transfers to that next recevier
    }

This algorithm ensures that there is at most 1 excessive receiver loafs about producing
unnecessary contention. When this receivers goes away, it wakes up another receiver. And
so on.