Move ShutdownHook to node-api. (#770)

node-api/src/main/kotlin/net/corda/nodeapi/internal/ShutdownHook.kt

@@ -0,0 +1,26 @@
+package net.corda.nodeapi.internal
+
+interface ShutdownHook {
+    /**
+     * Safe to call from the block passed into [addShutdownHook].
+     */
+    fun cancel()
+}
+
+/**
+ * The given block will run on most kinds of termination including SIGTERM, but not on SIGKILL.
+ * @return An object via which you can cancel the hook.
+ */
+fun addShutdownHook(block: () -> Unit): ShutdownHook {
+    val hook = Thread { block() }
+    val runtime = Runtime.getRuntime()
+    runtime.addShutdownHook(hook)
+    return object : ShutdownHook {
+        override fun cancel() {
+            // Allow the block to call cancel without causing IllegalStateException in the shutdown case:
+            if (Thread.currentThread() != hook) {
+                runtime.removeShutdownHook(hook)
+            }
+        }
+    }
+}