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>

testing/simulator/Cargo.toml

@@ -12,9 +12,10 @@ eth1 = {path = "../../beacon_node/eth1"}
 types = { path = "../../consensus/types" }
 validator_client = { path = "../../validator_client" }
 parking_lot = "0.11.0"
-futures = "0.3.5"
-tokio = "0.2.22"
+futures = "0.3.7"
+tokio = "0.3.2"
 eth1_test_rig = { path = "../eth1_test_rig" }
 env_logger = "0.7.1"
 clap = "2.33.3"
 rayon = "1.4.1"
+tokio-compat-02 = "0.1"