API for stratum server statistics.

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.

src/params.h

@@ -37,6 +37,7 @@ struct Params
 	std::string m_p2pPeerList;
 	std::string m_config;
 	std::string m_apiPath;
+	bool m_localStats = false;
 };
 
 } // namespace p2pool