Refactor price_info (#6003)

* refactor PriceInfo to use Fetch and reuse the client

* forget Fetch future to keep it running in the background

* update Debug message for price_info::Client

* wrap underlying errors in price_info client

* use debug_struct in price_info client debug implementation

* use global fetch service in price_info client

* rename gas_pricer parameter in RunCmd

* move price_info to its own crate

* fix price_info tests

* replace rustc_serialize with serde_json in price_info

* add documentation for price_info

* remove unused rustc-serialize dependency from ethcore

* fix price_info formatting

* re-export fetch crate in price_info

* remove unused cfg attributes in price_info

* add tests for price_info

util/fetch/src/client.rs

@@ -61,6 +61,14 @@ pub trait Fetch: Clone + Send + Sync + 'static {
 		f.boxed()
 	}
 
+	/// Spawn the future in context of this `Fetch` thread pool as "fire and forget", i.e. dropping this future without
+	/// canceling the underlying future.
+	/// Implementation is optional.
+	fn forget<F, I, E>(&self, _: F) where
+		F: Future<Item=I, Error=E> + Send + 'static,
+		I: Send + 'static,
+		E: Send + 'static {}
+
 	/// Fetch URL and get a future for the result.
 	/// Supports aborting the request in the middle of execution.
 	fn fetch_with_abort(&self, url: &str, abort: Abort) -> Self::Result;
@@ -149,6 +157,14 @@ impl Fetch for Client {
 		self.pool.spawn(f).boxed()
 	}
 
+	fn forget<F, I, E>(&self, f: F) where
+		F: Future<Item=I, Error=E> + Send + 'static,
+		I: Send + 'static,
+		E: Send + 'static,
+	{
+		self.pool.spawn(f).forget()
+	}
+
 	fn fetch_with_abort(&self, url: &str, abort: Abort) -> Self::Result {
 		debug!(target: "fetch", "Fetching from: {:?}", url);