Cleaning up the rest of the API functions.

- Removed all unused imports
 - Fixed random compiler errors
 - Removed all of the 'sucess_response' helpers.
 - Enabled all of the API endpoints again, wrapping in 'into_boxfut'
 - Tidied up /metrics endpoint
 - Added a 'body_text' part to ResponseBuilder, mainly for the Prometheus /metrics endpoint
 - Cleaned up the unnecessary helpers::* imports, to be more explicit.

beacon_node/rest_api/src/error.rs

@@ -1,7 +1,5 @@
 use crate::BoxFut;
-use futures::future::IntoFuture;
-use futures::Future;
-use hyper::{Body, Method, Request, Response, Server, StatusCode};
+use hyper::{Body, Response, StatusCode};
 use std::error::Error as StdError;
 
 #[derive(PartialEq, Debug, Clone)]
@@ -71,7 +69,7 @@ impl From<hyper::error::Error> for ApiError {
 }
 
 impl StdError for ApiError {
-    fn cause(&self) -> Option<&StdError> {
+    fn cause(&self) -> Option<&dyn StdError> {
         None
     }
 }