Improve tokio task execution (#1181)

* Add logging on shutdown

* Replace tokio::spawn with handle.spawn

* Upgrade tokio

* Add a task executor

* Beacon chain tasks use task executor

* Validator client tasks use task executor

* Rename runtime_handle to executor

* Add duration histograms; minor fixes

* Cleanup

* Fix logs

* Fix tests

* Remove random file

* Get enr dependency instead of libp2p

* Address some review comments

* Libp2p takes a TaskExecutor

* Ugly fix libp2p tests

* Move TaskExecutor to own file

* Upgrade Dockerfile rust version

* Minor fixes

* Revert "Ugly fix libp2p tests"

This reverts commit 58d4bb690f.

* Pretty fix libp2p tests

* Add spawn_without_exit; change Counter to Gauge

* Tidy

* Move log from RuntimeContext to TaskExecutor

* Fix errors

* Replace histogram with int_gauge for async tasks

* Fix todo

* Fix memory leak in test by exiting all spawned tasks at the end

beacon_node/client/src/lib.rs

@@ -25,8 +25,6 @@ pub struct Client<T: BeaconChainTypes> {
     network_globals: Option<Arc<NetworkGlobals<T::EthSpec>>>,
     http_listen_addr: Option<SocketAddr>,
     websocket_listen_addr: Option<SocketAddr>,
-    /// Exit channels will complete/error when dropped, causing each service to exit gracefully.
-    _exit_channels: Vec<tokio::sync::oneshot::Sender<()>>,
 }
 
 impl<T: BeaconChainTypes> Client<T> {