Automatically close useless connections (#816)

* Automatically close useless connections

* Add a timeout before dropping the connection

* Rework the timeout

* Use OR to combine the outcome

protocols/floodsub/src/handler.rs

@@ -148,6 +148,11 @@ where
     #[inline]
     fn inject_dial_upgrade_error(&mut self, _: Self::OutboundOpenInfo, _: ProtocolsHandlerUpgrErr<<Self::OutboundProtocol as OutboundUpgrade<Self::Substream>>::Error>) {}
 
+    #[inline]
+    fn connection_keep_alive(&self) -> bool {
+        !self.substreams.is_empty()
+    }
+
     #[inline]
     fn shutdown(&mut self) {
         self.shutting_down = true;