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/network.rs

@@ -1,9 +1,8 @@
-use crate::error::{ApiError, ApiResult};
-use crate::helpers::*;
+use crate::error::ApiResult;
 use crate::response_builder::ResponseBuilder;
 use crate::NetworkService;
 use beacon_chain::BeaconChainTypes;
-use eth2_libp2p::{Enr, Multiaddr, PeerId};
+use eth2_libp2p::{Multiaddr, PeerId};
 use hyper::{Body, Request};
 use std::sync::Arc;