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/eth1/Cargo.toml

@@ -10,10 +10,11 @@ toml = "0.5.6"
 web3 = "0.11.0"
 sloggers = "1.0.1"
 environment = { path = "../../lighthouse/environment" }
+tokio-compat-02 = "0.1"
 
 [dependencies]
 reqwest = { version = "0.10.8", features = ["native-tls-vendored"] }
-futures = { version = "0.3.5", features = ["compat"] }
+futures = { version = "0.3.7", features = ["compat"] }
 serde_json = "1.0.58"
 serde = { version = "1.0.116", features = ["derive"] }
 hex = "0.4.2"
@@ -25,7 +26,7 @@ tree_hash = "0.1.1"
 eth2_hashing = "0.1.0"
 parking_lot = "0.11.0"
 slog = "2.5.2"
-tokio = { version = "0.2.22", features = ["full"] }
+tokio = { version = "0.3.2", features = ["full"] }
 state_processing = { path = "../../consensus/state_processing" }
 libflate = "1.0.2"
 lighthouse_metrics = { path = "../../common/lighthouse_metrics"}