fix(protocols/kad): update KeepAlive::Until to preserve existing dead…

…line

Previous to this change if the ConnectionHandler::poll for kad was called more frequently than the
connection idle timeout the timeout would continually be pushed further into the feature. After this
change kad now will preserve the existing idle deadline.

protocols/kad/src/handler_priv.rs

@@ -759,7 +759,13 @@ where
 
         if self.outbound_substreams.is_empty() && self.inbound_substreams.is_empty() {
             // We destroyed all substreams in this function.
-            self.keep_alive = KeepAlive::Until(Instant::now() + self.config.idle_timeout);
+            self.keep_alive = if let KeepAlive::Until(time) = self.keep_alive {
+                // Preserve the existing idle timeout
+                KeepAlive::Until(time)
+            } else {
+                // Setup idle timeout
+                KeepAlive::Until(Instant::now() + self.config.idle_timeout)
+            };
         } else {
             self.keep_alive = KeepAlive::Yes;
         }