Close tasks channel after WaitGroup.Wait

[tklauser]

We could potentially run into an issue with writing to WorkerPool.tasks
in case the channel is closed concurrently:

  goroutine 1:                         goroutine 2:
   Submit                               Close
    -> wp.wg.Add(1)
    -> scheduled by runtime
                                        -> close(wp.tasks)
                                        -> scheduled by runtime
    -> wp.tasks <- &task{...}
    -> panic: write to closed channel

The situation can be provoked in some cases using the following test:

func TestSubmitCloseConcurrent(t *testing.T) {
       n := runtime.NumCPU()
       wp := New(n)
       for i := 0; i < n+2; i++ {
               go func() {
                       _ = wp.Submit("", func() error {
                               time.Sleep(time.Millisecond)
                               return nil
                       })
               }()
       }
       wp.Close()
}

Avoid the panic case by closing the tasks channel after waiting for the
WaitGroup.

The reproducing test is not added yet, as there seem to be other
concurrency issues present which the test triggers.

[rolinh]

Good catch, thanks!