Error from RPC send_response when request doesn't exist on the active inbound requests (#7663)

Lighthouse is currently loggign a lot errors in the `RPC` behaviour whenever a response is received for a request_id that no longer exists in active_inbound_requests. This is likely due to a data race or timing issue (e.g., the peer disconnecting before the response is handled).
This PR addresses that by removing the error logging from the RPC layer. Instead, RPC::send_response now simply returns an Err, shifting the responsibility to the main service. The main service can then determine whether the peer is still connected and only log an error if the peer remains connected.
Thanks @ackintosh for helping debug!

beacon_node/network/src/service.rs

@@ -11,6 +11,7 @@ use futures::channel::mpsc::Sender;
 use futures::future::OptionFuture;
 use futures::prelude::*;
 
+use lighthouse_network::rpc::methods::RpcResponse;
 use lighthouse_network::rpc::InboundRequestId;
 use lighthouse_network::rpc::RequestType;
 use lighthouse_network::service::Network;
@@ -627,10 +628,11 @@ impl<T: BeaconChainTypes> NetworkService<T> {
                 error,
                 inbound_request_id,
                 reason,
-            } => {
-                self.libp2p
-                    .send_error_response(peer_id, inbound_request_id, error, reason);
-            }
+            } => self.libp2p.send_response(
+                peer_id,
+                inbound_request_id,
+                RpcResponse::Error(error, reason.into()),
+            ),
             NetworkMessage::ValidationResult {
                 propagation_source,
                 message_id,