Remove secret_store runtimes. (#9888)

* Remove the independent runtimes from `KeyServerHttpListener` and
  `KeyServerCore` and instead require a `parity_runtime::Executor`
  to be passed upon creation of each.

* Remove the `threads` parameter from both `ClusterConfiguration` structs.

* Implement the `future::Executor` trait for `parity_runtime::Executor`.

* Update tests.
  - Update the `loop_until` function to instead use a oneshot to signal
    completion.
  - Modify the `make_key_servers` function to create and return a runtime.

util/runtime/src/lib.rs

@@ -16,8 +16,8 @@
 
 //! Tokio Runtime wrapper.
 
-extern crate futures;
-extern crate tokio;
+pub extern crate futures;
+pub extern crate tokio;
 
 use std::{fmt, thread};
 use std::sync::mpsc;
@@ -222,6 +222,24 @@ impl Executor {
 	}
 }
 
+impl<F: Future<Item = (), Error = ()> + Send + 'static> future::Executor<F> for Executor {
+	fn execute(&self, future: F) -> Result<(), future::ExecuteError<F>> {
+		match self.inner {
+			Mode::Tokio(ref executor) => executor.execute(future),
+			Mode::Sync => {
+				let _= future.wait();
+				Ok(())
+			},
+			Mode::ThreadPerFuture => {
+				thread::spawn(move || {
+					let _= future.wait();
+				});
+				Ok(())
+			},
+		}
+	}
+}
+
 /// A handle to a runtime. Dropping the handle will cause runtime to shutdown.
 pub struct RuntimeHandle {
 	close: Option<futures::Complete<()>>,