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/rlimit_unix.go

@@ -0,0 +1,40 @@
+// +build !windows
+
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package main
+
+import (
+	"syscall"
+)
+
+func setRlimit() error {
+	rlimit := uint64(100000)
+	if rlimit > 0 {
+		var limit syscall.Rlimit
+		if err := syscall.Getrlimit(syscall.RLIMIT_NOFILE, &limit); err != nil {
+			return err
+		}
+		if limit.Cur < rlimit {
+			limit.Cur = rlimit
+			if err := syscall.Setrlimit(syscall.RLIMIT_NOFILE, &limit); err != nil {
+				return err
+			}
+		}
+	}
+	return nil
+}