Correct issue with dialing peers (#2375)

The ordering of adding new peers to the peerdb and deciding when to dial them was not considered in a previous update.

This adds the condition that if a peer is not in the peer-db then it is an acceptable peer to dial.

This makes #2374 obsolete.

beacon_node/eth2_libp2p/src/peer_manager/peerdb.rs

@@ -160,6 +160,7 @@ impl<TSpec: EthSpec> PeerDB<TSpec> {
             self.connection_status(peer_id),
             Some(PeerConnectionStatus::Disconnected { .. })
                 | Some(PeerConnectionStatus::Unknown { .. })
+                | None
         ) && !self.is_banned_or_disconnected(peer_id)
     }