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/genesis/Cargo.toml

@@ -6,9 +6,10 @@ edition = "2018"
 
 [dev-dependencies]
 eth1_test_rig = { path = "../../testing/eth1_test_rig" }
+tokio-compat-02 = "0.1"
 
 [dependencies]
-futures = "0.3.5"
+futures = "0.3.7"
 types = { path = "../../consensus/types"}
 environment = { path = "../../lighthouse/environment"}
 eth1 = { path = "../eth1"}
@@ -18,7 +19,7 @@ merkle_proof = { path = "../../consensus/merkle_proof" }
 eth2_ssz = "0.1.2"
 eth2_hashing = "0.1.0"
 tree_hash = "0.1.1"
-tokio = { version = "0.2.22", features = ["full"] }
+tokio = { version = "0.3.2", features = ["full"] }
 parking_lot = "0.11.0"
 slog = "2.5.2"
 exit-future = "0.2.0"