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/eth2_libp2p/src/lib.rs

@@ -7,6 +7,8 @@ extern crate lazy_static;
 
 pub mod behaviour;
 mod config;
+
+#[allow(clippy::mutable_key_type)] // PeerId in hashmaps are no longer permitted by clippy
 pub mod discovery;
 mod metrics;
 mod peer_manager;
@@ -64,6 +66,7 @@ pub use config::Config as NetworkConfig;
 pub use config::{GossipsubConfig, GossipsubConfigBuilder, GossipsubMessage};
 pub use discovery::{CombinedKeyExt, EnrExt, Eth2Enr};
 pub use discv5;
+pub use libp2p::bandwidth::BandwidthSinks;
 pub use libp2p::gossipsub::{MessageAcceptance, MessageId, Topic, TopicHash};
 pub use libp2p::{core::ConnectedPoint, PeerId, Swarm};
 pub use libp2p::{multiaddr, Multiaddr};