Handle RPC requests in the substrate-service (#2866)

* Rework RPC queries

* Remove SyncProvider trait

* Fix RPC tests

substrate/core/rpc/src/helpers.rs

@@ -14,6 +14,21 @@
 // You should have received a copy of the GNU General Public License
 // along with Substrate.  If not, see <http://www.gnu.org/licenses/>.
 
+use futures::{prelude::*, sync::oneshot};
+
+/// Wraps around `oneshot::Receiver` and adjusts the error type to produce an internal error if the
+/// sender gets dropped.
+pub struct Receiver<T>(pub oneshot::Receiver<T>);
+
+impl<T> Future for Receiver<T> {
+	type Item = T;
+	type Error = jsonrpc_core::Error;
+
+	fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
+		self.0.poll().map_err(|_| jsonrpc_core::Error::internal_error())
+	}
+}
+
 /// Unwraps the trailing parameter or falls back with the closure result.
 pub fn unwrap_or_else<F, H, E>(or_else: F, optional: Option<H>) -> Result<H, E> where
 	F: FnOnce() -> Result<H, E>,