introduce an async version of stratum server. (#2468)

It seems that current approach stops workring after amount of parallel
connection exceeds ~500. This PR introduces an 'async' feature which
enable asyncronius, tokio based implementation of stratum server.

* fix bug with passing current_difficulty in Handler

I passed in Handler current_difficulty as an u64, so it was copied
and all later changes were ignored. In this commit I pass referece on
RwLock with current_difficulty.

* fix crash in build_block

* improve  error handling

* print error message on unknown error

* replace original server by tokio based one

* fixes after review

* reduce sleep time in mail loop to 5ms

servers/Cargo.toml

@@ -24,6 +24,7 @@ serde_json = "1"
 chrono = "0.4.4"
 bufstream = "~0.1"
 jsonrpc-core = "~8.0"
+tokio =  "0.1.11"
 
 grin_api = { path = "../api", version = "1.0.1" }
 grin_chain = { path = "../chain", version = "1.0.1" }

servers/src/grin/server.rs

@@ -302,12 +302,12 @@ impl Server {
 			self.chain.clone(),
 			self.tx_pool.clone(),
 			self.verifier_cache.clone(),
+			self.state_info.stratum_stats.clone(),
 		);
-		let stratum_stats = self.state_info.stratum_stats.clone();
 		let _ = thread::Builder::new()
 			.name("stratum_server".to_string())
 			.spawn(move || {
-				stratum_server.run_loop(stratum_stats, edge_bits as u32, proof_size, sync_state);
+				stratum_server.run_loop(edge_bits as u32, proof_size, sync_state);
 			});
 	}