To reproduce the deadlock, put `runtime.Gosched()` right after
`workersReady <- 1`.  The `q.authCond.Signal()` is the being sent
without any worker waiting at `q.authCond.Wait()`.  Therefore all
workers block at `q.authCond.Wait()` and the main goroutine blocks at
`wg.Wait()` (deadlock).

Use case 1, `q.transferables` is empty:

  `<-workersReady` is not (endlessly) blocking, because the additional
  `go func` closes the channel.  The `q.authCond.Signal()` does not do
  anything.

Use case 2, `q.transferables` is not empty:

  `<-workersReady` in combination with `q.authCond.L.Lock()` blocks
  until at least one worker is waiting (`q.authCond.Wait()`).
  Therefore the `q.authCond.Signal()` signals one of the goroutines to
  run.

  The size of the channel `workersReady` makes sure that every worker
  that passed `q.authCond.L.Lock()` before the main goroutine does not
  block at `workersReady <- 1`.