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/network/Cargo.toml

@@ -11,6 +11,8 @@ lazy_static = "1.4.0"
 matches = "0.1.8"
 tempfile = "3.1.0"
 exit-future = "0.2.0"
+slog-term = "2.6.0"
+slog-async = "2.5.0"
 
 [dependencies]
 beacon_chain =  { path = "../beacon_chain" }
@@ -25,9 +27,9 @@ hex = "0.4.2"
 eth2_ssz = "0.1.2"
 eth2_ssz_types = { path =  "../../consensus/ssz_types" }
 tree_hash = "0.1.1"
-futures = "0.3.5"
+futures = "0.3.7"
 error-chain = "0.12.4"
-tokio = { version = "0.2.22", features = ["full"] }
+tokio = { version = "0.3.2", features = ["full"] }
 parking_lot = "0.11.0"
 smallvec = "1.4.2"
 rand = "0.7.3"