(RFR) Profiling Improvements

* Adds a `JsContext` which wraps Otto and the Handlers.
* Pool the `JsContext` so that they are re-used between requests.
* Add `-cpuprofile` option and catch `ctrl+c` to write it out to disk.
* Set `GOMAXPROCS` based on `runtime.NumCPU` if the environment variable was not set.

TODO:
- [ ] Benchmarks vs `master`