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>

lighthouse/environment/tests/environment_builder.rs

@@ -7,7 +7,7 @@ use types::{V012LegacyEthSpec, YamlConfig};
 
 fn builder() -> EnvironmentBuilder<V012LegacyEthSpec> {
     EnvironmentBuilder::v012_legacy()
-        .single_thread_tokio_runtime()
+        .multi_threaded_tokio_runtime()
         .expect("should set runtime")
         .null_logger()
         .expect("should set logger")