Stop processing the request after failing an overloaded connection

zebra-network's Connection expects that `fail_with` is only called once
per connection, but the overload handling code continues to process the
current request after an overload error, potentially leading to further
failures.

Closes ZcashFoundation#1599

zebra-network/src/peer/connection.rs

@@ -840,6 +840,7 @@ where
         if self.svc.ready_and().await.is_err() {
             // Treat all service readiness errors as Overloaded
             self.fail_with(PeerError::Overloaded);
+            return;
         }
 
         let rsp = match self.svc.call(req).await {