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/genesis/Cargo.toml

@@ -18,7 +18,7 @@ merkle_proof = { path = "../../consensus/merkle_proof" }
 eth2_ssz = "0.1.2"
 eth2_hashing = "0.1.0"
 tree_hash = "0.1.0"
-tokio = { version = "0.2.20", features = ["full"] }
+tokio = { version = "0.2.21", features = ["full"] }
 parking_lot = "0.10.2"
 slog = "2.5.2"
 exit-future = "0.2.0"