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/peer_manager/mod.rs

@@ -27,6 +27,7 @@ pub use libp2p::core::{identity::Keypair, Multiaddr};
 pub mod client;
 mod peer_info;
 mod peer_sync_status;
+#[allow(clippy::mutable_key_type)] // PeerId in hashmaps are no longer permitted by clippy
 mod peerdb;
 pub(crate) mod score;
 
@@ -639,6 +640,7 @@ impl<TSpec: EthSpec> PeerManager<TSpec> {
     /// with a new `PeerId` which involves a discovery routing table lookup. We could dial the
     /// multiaddr here, however this could relate to duplicate PeerId's etc. If the lookup
     /// proves resource constraining, we should switch to multiaddr dialling here.
+    #[allow(clippy::mutable_key_type)]
     fn peers_discovered(&mut self, results: HashMap<PeerId, Option<Instant>>) {
         let mut to_dial_peers = Vec::new();