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/simulator/src/eth1_sim.rs

@@ -197,6 +197,12 @@ pub fn run_eth1_sim(matches: &ArgMatches) -> Result<(), String> {
         Ok::<(), String>(())
     };
 
-    env.runtime().block_on(main_future).unwrap();
+    env.runtime()
+        .block_on(tokio_compat_02::FutureExt::compat(main_future))
+        .unwrap();
+
+    env.fire_signal();
+    env.shutdown_on_idle();
+
     Ok(())
 }