Add cio.Load for loading io set

This adds a `Load` Opt for cio to load a tasks io/fifos without
attaching or starting the copy routines.

It adds the load method in `ctr` by default so that fifos or other IO
are removed from disk on delete methods inbetween command runs.  It is
not the default for all task loads for backwards compat. and a user may
want to keep io around to reuse or if log files are used.

Fixes #2421

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

cmd/ctr/commands/tasks/delete.go

@@ -18,6 +18,7 @@ package tasks
 
 import (
 	"github.com/containerd/containerd"
+	"github.com/containerd/containerd/cio"
 	"github.com/containerd/containerd/cmd/ctr/commands"
 	"github.com/urfave/cli"
 )
@@ -42,8 +43,7 @@ var deleteCommand = cli.Command{
 		if err != nil {
 			return err
 		}
-
-		task, err := container.Task(ctx, nil)
+		task, err := container.Task(ctx, cio.Load)
 		if err != nil {
 			return err
 		}