Add density stress test

Running the density tool will report Pss and Rss total and per container
values for shim memory usage. Values are reported in KB.

```bash
containerd-stress density --count 500
INFO[0000] pulling docker.io/library/alpine:latest
INFO[0000] generating spec from image
{"pss":421188,"rss":2439688,"pssPerContainer":842,"rssPerContainer":4879}
```

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

cmd/containerd-stress/main.go

@@ -55,6 +55,11 @@ func init() {
 	binarySizeGauge = ns.NewLabeledGauge("binary_size", "Binary size of compiled binaries", metrics.Bytes, "name")
 	errCounter = ns.NewLabeledCounter("errors", "Errors encountered running the stress tests", "err")
 	metrics.Register(ns)
+
+	// set higher ulimits
+	if err := setRlimit(); err != nil {
+		panic(err)
+	}
 }
 
 type run struct {
@@ -150,6 +155,9 @@ func main() {
 		}
 		return nil
 	}
+	app.Commands = []cli.Command{
+		densityCommand,
+	}
 	app.Action = func(context *cli.Context) error {
 		config := config{
 			Address:     context.GlobalString("address"),