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/containers/containers.go

@@ -25,6 +25,7 @@ import (
 	"text/tabwriter"
 
 	"github.com/containerd/containerd"
+	"github.com/containerd/containerd/cio"
 	"github.com/containerd/containerd/cmd/ctr/commands"
 	"github.com/containerd/containerd/cmd/ctr/commands/run"
 	"github.com/containerd/containerd/log"
@@ -162,7 +163,6 @@ var deleteCommand = cli.Command{
 				log.G(ctx).WithError(err).Errorf("failed to delete container %q", arg)
 			}
 		}
-
 		return exitErr
 	},
 }
@@ -172,7 +172,7 @@ func deleteContainer(ctx context.Context, client *containerd.Client, id string,
 	if err != nil {
 		return err
 	}
-	task, err := container.Task(ctx, nil)
+	task, err := container.Task(ctx, cio.Load)
 	if err != nil {
 		return container.Delete(ctx, opts...)
 	}