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/network/src/lib.rs

@@ -3,15 +3,18 @@ extern crate lazy_static;
 
 /// This crate provides the network server for Lighthouse.
 pub mod error;
+#[allow(clippy::mutable_key_type)] // PeerId in hashmaps are no longer permitted by clippy
 pub mod service;
 
 mod attestation_service;
 mod beacon_processor;
+#[allow(clippy::mutable_key_type)] // PeerId in hashmaps are no longer permitted by clippy
 mod metrics;
 mod nat;
 mod persisted_dht;
 mod router;
 mod status;
+#[allow(clippy::mutable_key_type)] // PeerId in hashmaps are no longer permitted by clippy
 mod sync;
 
 pub use eth2_libp2p::NetworkConfig;