Tests/benchmarks should clean up after themselves

[mreiferson]

There are many tests that drop tempfiles directly into os.TempDir() and don't delete them. Instead, these tests should do this:

tempDir, err := ioutil.TempDir("", "nsq-some-desc-here")
if err != nil {
  t.Fatal(err)
}
defer os.RemoveAll(tempDir)

I discovered this by finding that /tmp has 21GB of nsq data data in it.

I'll try to send over a PR for this.

[mreiferson]

RFR @jehiah @cespare