db/update: cancel the update on shutdown

src/db/update/Service.hxx

@@ -64,6 +64,8 @@ public:
 		      Storage &_storage,
 		      DatabaseListener &_listener);
 
+	~UpdateService();
+
 	/**
 	 * Returns a non-zero job id when we are currently updating
 	 * the database.
@@ -82,6 +84,12 @@ public:
 	gcc_nonnull_all
 	unsigned Enqueue(const char *path, bool discard);
 
+	/**
+	 * Clear the queue and cancel the current update.  Does not
+	 * wait for the thread to exit.
+	 */
+	void CancelAllAsync();
+
 private:
 	/* virtual methods from class DeferredMonitor */
 	virtual void RunDeferred() override;