Add an hrtime Executor

[dantleech]

Create an hrtime executor, or rather a more flexible executor that can be configured.

• Use hrtime or use it by default
• Configuration for:
  • Where before methods are called
  • If the sample is taken from within / without the for loop
  • If GC etc should be enabled
  • ...

---

Proposition: allow executors to be configured explicitly using "stages"

    "runner.executors": {
        "hrtime": {
            "executor": "custom",
            "stages": [
                "disableGc",
                "bootstrap",
                "warmup",
                {
                    "stage": "hrtimer",
                    "children": [
                        {
                            "stage": "revsIteration",
                            "children": [
                                "callSubject"
                            ]
                        }
                    ]
                }
            ]
        }
    },