Getting regular endpoint functions to return futures.

- Wrapped endpoint functions in new into_boxfut function
 - Undid changes to Network API service, now returning ApiResult again.
 - Cleaning up of functions, and removal of success_response functions in updated endpoints.
 - A bunch of other clean-ups.

beacon_node/rest_api/src/error.rs

@@ -1,4 +1,6 @@
 use crate::BoxFut;
+use futures::future::IntoFuture;
+use futures::Future;
 use hyper::{Body, Method, Request, Response, Server, StatusCode};
 use std::error::Error as StdError;