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>

beacon_node/websocket_server/src/lib.rs

@@ -79,11 +79,11 @@ pub fn start_server<T: EthSpec>(
 
     // Place a future on the handle that will shutdown the websocket server when the
     // application exits.
-    executor.runtime_handle().spawn(exit_future);
+
+    executor.spawn(exit_future, "Websocket exit");
 
     let log_inner = log.clone();
-
-    let _ = std::thread::spawn(move || match server.run() {
+    let server_future = move || match server.run() {
         Ok(_) => {
             debug!(
                 log_inner,
@@ -97,7 +97,9 @@ pub fn start_server<T: EthSpec>(
                 "error" => format!("{:?}", e)
             );
         }
-    });
+    };
+
+    executor.spawn_blocking(server_future, "Websocket server");
 
     info!(
         log,