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>

remote_signer/client/Cargo.toml

@@ -9,7 +9,7 @@ clap = "2.33.3"
 client_backend = { path = "../backend", package = "remote_signer_backend" }
 environment = { path = "../../lighthouse/environment" }
 futures = "0.3.6"
-hyper = "0.13.8"
+hyper = { git = "https://github.com/sigp/hyper", branch = "lighthouse" }
 lazy_static = "1.4.0"
 regex = "1.3.9"
 serde = { version = "1.0.116", features = ["derive"] }