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/src/lib.rs

@@ -72,10 +72,8 @@ impl<E: EthSpec> ProductionBeaconNode<E> {
         let client_genesis = client_config.genesis.clone();
         let store_config = client_config.store.clone();
         let log = context.log().clone();
-
         let db_path = client_config.create_db_path()?;
         let freezer_db_path_res = client_config.create_freezer_db_path();
-
         let executor = context.executor.clone();
 
         let builder = ClientBuilder::new(context.eth_spec_instance.clone())