Ignore RPC messages of disconnected peers and remove old peers based on disconnection time (#1854)

## Issue Addressed

NA

## Proposed Changes

Lets the networking behavior ignore messages of peers that are not connected. Furthermore, old peers are not removed from the peerdb based on score anymore but based on the disconnection time.

beacon_node/eth2_libp2p/src/behaviour/mod.rs

@@ -589,6 +589,17 @@ impl<TSpec: EthSpec> Behaviour<TSpec> {
 
     fn on_rpc_event(&mut self, message: RPCMessage<TSpec>) {
         let peer_id = message.peer_id;
+
+        if !self.peer_manager.is_connected(&peer_id) {
+            //ignore this event
+            debug!(
+                self.log,
+                "Ignoring rpc message of disconnected peer";
+                "peer" => peer_id.to_string()
+            );
+            return;
+        }
+
         let handler_id = message.conn_id;
         // The METADATA and PING RPC responses are handled within the behaviour and not propagated
         match message.event {