New Chain struct acting as facade for all chain operations

Introduce a new Chain struct that maintains the current head,
acts as facade to the store and the block processing pipeline
and handles all appropriate locking.

All higher level components, like the overall server, REST APIs
and miner have been update to only depend on Chain. Cleaned up
some duplication and tests now that there's a single entry point
to all blockchain operations.

src/bin/grin.rs

@@ -254,9 +254,8 @@ fn wallet_command(wallet_args: &ArgMatches) {
 		wallet_config.check_node_api_http_addr = sa.to_string().clone();
 	}
 
-	
 	match wallet_args.subcommand() {
-		
+
 		("receive", Some(receive_args)) => {
 			if let Some(f) = receive_args.value_of("input") {
 				let mut file = File::open(f).expect("Unable to open transaction file.");
@@ -264,12 +263,14 @@ fn wallet_command(wallet_args: &ArgMatches) {
 				file.read_to_string(&mut contents).expect("Unable to read transaction file.");
 				wallet::receive_json_tx(&wallet_config, &key, contents.as_str()).unwrap();
 			} else {
-				info!("Starting the Grin wallet receiving daemon at {}...", wallet_config.api_http_addr);
+				info!("Starting the Grin wallet receiving daemon at {}...",
+				      wallet_config.api_http_addr);
 				let mut apis = api::ApiServer::new("/v1".to_string());
-				apis.register_endpoint("/receive".to_string(), wallet::WalletReceiver { 
-					key: key,
-					config: wallet_config
-				});
+				apis.register_endpoint("/receive".to_string(),
+				                       wallet::WalletReceiver {
+					                       key: key,
+					                       config: wallet_config,
+				                       });
 				apis.start(addr).unwrap_or_else(|e| {
 					error!("Failed to start Grin wallet receiver: {}.", e);
 				});
@@ -304,7 +305,7 @@ fn read_config() -> grin::ServerConfig {
 
 fn default_config() -> grin::ServerConfig {
 	grin::ServerConfig {
-    test_mode: true,
+		test_mode: true,
 		seeding_type: grin::Seeding::WebStatic,
 		..Default::default()
 	}