Upgrade to tokio 0.3 (#1839)

## Description

This PR updates Lighthouse to tokio 0.3. It includes a number of dependency updates and some structural changes as to how we create and spawn tasks.

This also brings with it a number of various improvements:

- Discv5 update
- Libp2p update
- Fix for recompilation issues
- Improved UPnP port mapping handling
- Futures dependency update
- Log downgrade to traces for rejecting peers when we've reached our max



Co-authored-by: blacktemplar <blacktemplar@a1.net>

testing/node_test_rig/src/lib.rs

@@ -86,6 +86,7 @@ pub fn testing_client_config() -> ClientConfig {
     // Setting ports to `0` means that the OS will choose some available port.
     client_config.network.libp2p_port = 0;
     client_config.network.discovery_port = 0;
+    client_config.network.upnp_enabled = false;
     client_config.http_api.enabled = true;
     client_config.http_api.listen_port = 0;
     client_config.websocket_server.enabled = true;