Discovery v5.1 (#1786)

## Overview 

This updates lighthouse to discovery v5.1

Note: This makes lighthouse's discovery not compatible with any previous version. Lighthouse cannot discover peers or send/receive ENR's from any previous version. This is a breaking change. 

This resolves #1605

beacon_node/eth2_libp2p/src/discovery/mod.rs

@@ -240,7 +240,7 @@ impl<TSpec: EthSpec> Discovery<TSpec> {
 
         while let Some((result, original_addr)) = fut_coll.next().await {
             match result {
-                Ok(Some(enr)) => {
+                Ok(enr) => {
                     debug!(
                         log,
                         "Adding node to routing table";
@@ -259,9 +259,6 @@ impl<TSpec: EthSpec> Discovery<TSpec> {
                         )
                     });
                 }
-                Ok(None) => {
-                    error!(log, "No ENR found for MultiAddr"; "addr" => original_addr.to_string())
-                }
                 Err(e) => {
                     error!(log, "Error getting mapping to ENR"; "multiaddr" => original_addr.to_string(), "error" => e.to_string())
                 }