Set worker threads for async update task

This is necessary for correct state reporting.

demo/grouper/midpoint-objects-manual/tasks/task-async-update-grouper.xml

@@ -25,7 +25,7 @@
 	<name>Grouper async updates</name>
 	<extension xmlns:mext="http://midpoint.evolveum.com/xml/ns/public/model/extension-3"
 			   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="c:ExtensionType">
-		<!-- ... -->
+		<mext:workerThreads>1</mext:workerThreads>
 	</extension>
 	<taskIdentifier>1552664339630-0-2</taskIdentifier>
 	<ownerRef oid="00000000-0000-0000-0000-000000000002" relation="org:default" type="c:UserType">