Make Wait() async

In all of the examples, its recommended to call `Wait()` before starting
a process/task.
Since `Wait()` is a blocking call, this means it must be called from a
goroutine like so:

```go
statusC := make(chan uint32)
go func() {
  status, err := task.Wait(ctx)
  if err != nil {
    // handle async err
  }

  statusC <- status
}()

task.Start(ctx)
<-statusC
```

This means there is a race here where there is no guarentee when the
goroutine is going to be scheduled, and even a bit more since this
requires an RPC call to be made.
In addition, this code is very messy and a common pattern for any caller
using Wait+Start.

Instead, this changes `Wait()` to use an async model having `Wait()`
return a channel instead of the code itself.
This ensures that when `Wait()` returns that the client has a handle on
the event stream (already made the RPC request) before returning and
reduces any sort of race to how the stream is handled by grpc since we
can't guarentee that we have a goroutine running and blocked on
`Recv()`.

Making `Wait()` async also cleans up the code in the caller drastically:

```go
statusC, err := task.Wait(ctx)
if err != nil {
  return err
}

task.Start(ctx)

status := <-statusC
if status.Err != nil {
  return err
}
```

No more spinning up goroutines and more natural error
handling for the caller.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

container_linux_test.go

@@ -57,14 +57,11 @@ func TestContainerUpdate(t *testing.T) {
 	}
 	defer task.Delete(ctx)
 
-	statusC := make(chan uint32, 1)
-	go func() {
-		status, err := task.Wait(ctx)
-		if err != nil {
-			t.Error(err)
-		}
-		statusC <- status
-	}()
+	statusC, err := task.Wait(ctx)
+	if err != nil {
+		t.Error(err)
+		return
+	}
 
 	// check that the task has a limit of 32mb
 	cgroup, err := cgroups.Load(cgroups.V1, cgroups.PidPath(int(task.Pid())))
@@ -157,14 +154,12 @@ func TestShimInCgroup(t *testing.T) {
 	}
 	defer task.Delete(ctx)
 
-	statusC := make(chan uint32, 1)
-	go func() {
-		status, err := task.Wait(ctx)
-		if err != nil {
-			t.Error(err)
-		}
-		statusC <- status
-	}()
+	statusC, err := task.Wait(ctx)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+
 	// check to see if the shim is inside the cgroup
 	processes, err := cg.Processes(cgroups.Devices, false)
 	if err != nil {
@@ -221,17 +216,11 @@ func TestDaemonRestart(t *testing.T) {
 	}
 	defer task.Delete(ctx)
 
-	synC := make(chan struct{})
-	statusC := make(chan uint32, 1)
-	go func() {
-		synC <- struct{}{}
-		status, err := task.Wait(ctx)
-		if err == nil {
-			t.Errorf(`first task.Wait() should have failed with "transport is closing"`)
-		}
-		statusC <- status
-	}()
-	<-synC
+	statusC, err := task.Wait(ctx)
+	if err != nil {
+		t.Error(err)
+		return
+	}
 
 	if err := task.Start(ctx); err != nil {
 		t.Error(err)
@@ -242,7 +231,10 @@ func TestDaemonRestart(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	<-statusC
+	status := <-statusC
+	if status.Err == nil {
+		t.Errorf(`first task.Wait() should have failed with "transport is closing"`)
+	}
 
 	waitCtx, waitCancel := context.WithTimeout(ctx, 2*time.Second)
 	serving, err := client.IsServing(waitCtx)
@@ -251,15 +243,11 @@ func TestDaemonRestart(t *testing.T) {
 		t.Fatalf("containerd did not start within 2s: %v", err)
 	}
 
-	go func() {
-		synC <- struct{}{}
-		status, err := task.Wait(ctx)
-		if err != nil {
-			t.Error(err)
-		}
-		statusC <- status
-	}()
-	<-synC
+	statusC, err = task.Wait(ctx)
+	if err != nil {
+		t.Error(err)
+		return
+	}
 
 	if err := task.Kill(ctx, syscall.SIGKILL); err != nil {
 		t.Fatal(err)