Add pause/unpause implementation

This adds pause and unpause to containerd's execution service and the
same commands to the `ctr` client.

Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com>

cmd/ctr/pause.go

@@ -0,0 +1,29 @@
+package main
+
+import (
+	gocontext "context"
+	"errors"
+
+	"github.com/containerd/containerd/api/services/execution"
+	"github.com/urfave/cli"
+)
+
+var pauseCommand = cli.Command{
+	Name:      "pause",
+	Usage:     "pause an existing container",
+	ArgsUsage: "CONTAINER",
+	Action: func(context *cli.Context) error {
+		containers, err := getExecutionService(context)
+		if err != nil {
+			return err
+		}
+		id := context.Args().First()
+		if id == "" {
+			return errors.New("container id must be provided")
+		}
+		_, err = containers.Pause(gocontext.Background(), &execution.PauseRequest{
+			ID: id,
+		})
+		return err
+	},
+}