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/client/src/notifier.rs

@@ -7,7 +7,7 @@ use slog::{debug, error, info, warn};
 use slot_clock::SlotClock;
 use std::sync::Arc;
 use std::time::{Duration, Instant};
-use tokio::time::delay_for;
+use tokio::time::sleep;
 use types::{EthSpec, Slot};
 
 /// Create a warning log whenever the peer count is at or below this value.
@@ -56,7 +56,7 @@ pub fn spawn_notifier<T: BeaconChainTypes>(
                         "peers" => peer_count_pretty(network.connected_peers()),
                         "wait_time" => estimated_time_pretty(Some(next_slot.as_secs() as f64)),
                     );
-                    delay_for(slot_duration).await;
+                    sleep(slot_duration).await;
                 }
                 _ => break,
             }