Upgrade discovery and restructure task execution (#1693)

* Initial rebase

* Remove old code

* Correct release tests

* Rebase commit

* Remove eth2-testnet dep on eth2libp2p

* Remove crates lost in rebase

* Remove unused dep

beacon_node/network/src/service/tests.rs

@@ -41,7 +41,7 @@ mod tests {
 
         let (signal, exit) = exit_future::signal();
         let (shutdown_tx, _) = futures::channel::mpsc::channel(1);
-        let executor = environment::TaskExecutor::new(
+        let executor = task_executor::TaskExecutor::new(
             runtime.handle().clone(),
             exit,
             log.clone(),