Improve Lighthouse Connectivity Via ENR TCP Update (#4057)

Currently Lighthouse will remain uncontactable if users port forward a port that is not the same as the one they are listening on. 

For example, if Lighthouse runs with port 9000 TCP/UDP locally but a router is configured to pass 9010 externally to the lighthouse node on 9000, other nodes on the network will not be able to reach the lighthouse node. 

This occurs because Lighthouse does not update its ENR TCP port on external socket discovery. The intention was always that users should use `--enr-tcp-port` to customise this, but this is non-intuitive. 

The difficulty arises because we have no discovery mechanism to find our external TCP port. If we discovery a new external UDP port, we must guess what our external TCP port might be. This PR assumes the external TCP port is the same as the external UDP port (which may not be the case) and thus updates the TCP port along with the UDP port if the `--enr-tcp-port` flag is not set. 

Along with this PR, will be added documentation to the Lighthouse book so users can correctly understand and configure their ENR to maximize Lighthouse's connectivity. 

This relies on https://github.com/sigp/discv5/pull/166 and we should wait for a new release in discv5 before adding this PR.

boot_node/src/server.rs

@@ -44,7 +44,7 @@ pub async fn run<T: EthSpec>(config: BootNodeConfig<T>, log: slog::Logger) {
     info!(log, "Contact information"; "multiaddrs" => ?local_enr.multiaddr_p2p());
 
     // construct the discv5 server
-    let mut discv5 = Discv5::new(local_enr.clone(), local_key, discv5_config).unwrap();
+    let mut discv5: Discv5 = Discv5::new(local_enr.clone(), local_key, discv5_config).unwrap();
 
     // If there are any bootnodes add them to the routing table
     for enr in boot_nodes {