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/websocket_server/Cargo.toml

@@ -7,11 +7,11 @@ edition = "2018"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
-futures = "0.3.5"
+futures = "0.3.7"
 serde = "1.0.116"
 serde_derive = "1.0.116"
 slog = "2.5.2"
-tokio = { version = "0.2.22", features = ["full"] }
+tokio = { version = "0.3.2", features = ["full"] }
 types = { path = "../../consensus/types" }
 ws = "0.9.1"
 task_executor = { path = "../../common/task_executor" }