Users can pass `--stratum-api` to command line to enable a file called `local/stats` in `--data-api` directory. `local/stats` contains statistics that `StratumServer status` provides but in JSON format. It is currently setup that `local/stats` is updated each time a new job is sent to a worker.
- Proper shutdown sequence, uv_close() must be called from the event loop thread
- Moved start_listening() to child class constructor because it must be ready before it can listen on sockets
- Added simple memory leak detector for Windows. Linux users can enjoy the leak sanitizer
