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>

validator_client/src/block_service.rs

@@ -188,21 +188,22 @@ impl<T: SlotClock + 'static, E: EthSpec> BlockService<T, E> {
             )
         }
 
-        proposers.into_iter().for_each(|validator_pubkey| {
+        for validator_pubkey in proposers {
             let service = self.clone();
             let log = log.clone();
-            self.inner.context.executor.runtime_handle().spawn(
+            self.inner.context.executor.spawn(
                 service
                     .publish_block(slot, validator_pubkey)
-                    .map_err(move |e| {
+                    .unwrap_or_else(move |e| {
                         crit!(
                             log,
                             "Error whilst producing block";
                             "message" => e
-                        )
+                        );
                     }),
+                "block service",
             );
-        });
+        }
 
         Ok(())
     }