light: integrate with sync + serve_light CLI

parity/cli/config.full.toml

@@ -32,6 +32,7 @@ warp = true
 allow_ips = "all"
 snapshot_peers = 0
 max_pending_peers = 64
+serve_light = true
 
 reserved_only = false
 reserved_peers = "./path_to_file"

parity/cli/mod.rs

@@ -135,6 +135,8 @@ usage! {
 		flag_reserved_only: bool = false,
 			or |c: &Config| otry!(c.network).reserved_only.clone(),
 		flag_no_ancient_blocks: bool = false, or |_| None,
+		flag_serve_light: bool = false,
+			or |c: &Config| otry!(c.network).serve_light.clone(),
 
 		// -- API and Console Options
 		// RPC
@@ -334,6 +336,7 @@ struct Network {
 	node_key: Option<String>,
 	reserved_peers: Option<String>,
 	reserved_only: Option<bool>,
+	serve_light: Option<bool>,
 }
 
 #[derive(Default, Debug, PartialEq, RustcDecodable)]
@@ -543,6 +546,7 @@ mod tests {
 			flag_reserved_peers: Some("./path_to_file".into()),
 			flag_reserved_only: false,
 			flag_no_ancient_blocks: false,
+			flag_serve_light: true,
 
 			// -- API and Console Options
 			// RPC
@@ -713,6 +717,7 @@ mod tests {
 				node_key: None,
 				reserved_peers: Some("./path/to/reserved_peers".into()),
 				reserved_only: Some(true),
+				serve_light: None,
 			}),
 			rpc: Some(Rpc {
 				disable: Some(true),

parity/cli/usage.txt

@@ -97,6 +97,7 @@ Networking Options:
   --max-pending-peers NUM  Allow up to NUM pending connections. (default: {flag_max_pending_peers})
   --no-ancient-blocks      Disable downloading old blocks after snapshot restoration
                            or warp sync. (default: {flag_no_ancient_blocks})
+  --serve-light            Experimental: Serve light client peers. (default: {flag_serve_light})
 
 API and Console Options:
   --no-jsonrpc             Disable the JSON-RPC API server. (default: {flag_no_jsonrpc})