Augment every task spawned by Service with on_exit (#3581)

* Augment every task spawned by Service with `on_exit`

* Add CI test that the node exits

substrate/ci/check_for_exit.sh

@@ -0,0 +1,16 @@
+#!/usr/bin/env bash
+
+# Script that checks that a node exits after `SIGINT` was send.
+
+set -e
+
+cargo build --release
+./target/release/substrate --dev &
+PID=$!
+
+# Let the chain running for 60 seconds
+sleep 60
+
+# Send `SIGINT` and give the process 30 seconds to end
+kill -INT $PID
+timeout 30 tail --pid=$PID -f /dev/null