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>

common/eth2_testnet_config/build.rs

@@ -52,8 +52,11 @@ fn uncompress_state(testnet: &Eth2NetArchiveAndDirectory<'static>) -> Result<(),
             .map_err(|e| format!("Error writing file {:?}: {}", path, e))?;
     } else {
         // Create empty genesis.ssz if genesis is unknown
-        File::create(testnet.dir().join(GENESIS_FILE_NAME))
-            .map_err(|e| format!("Failed to create {}: {}", GENESIS_FILE_NAME, e))?;
+        let genesis_file = testnet.dir().join(GENESIS_FILE_NAME);
+        if !genesis_file.exists() {
+            File::create(genesis_file)
+                .map_err(|e| format!("Failed to create {}: {}", GENESIS_FILE_NAME, e))?;
+        }
     }
 
     Ok(())