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/beacon.rs

@@ -1,6 +1,6 @@
 use crate::helpers::*;
 use crate::response_builder::ResponseBuilder;
-use crate::{ApiError, ApiResult, BoxFut, NetworkService, UrlQuery};
+use crate::{ApiError, ApiResult, UrlQuery};
 use beacon_chain::{BeaconChain, BeaconChainTypes};
 use hyper::{Body, Request};
 use serde::Serialize;