Clear statistics for iterative scripting tasks

When starting, these tasks started progress anew, but other statistics
were accumulating.

This resolves MID-6488.

(cherry picked from commit 5e6f257)

model/model-impl/src/main/java/com/evolveum/midpoint/model/impl/scripting/IterativeScriptExecutionTaskHandler.java

@@ -43,7 +43,8 @@ public class IterativeScriptExecutionTaskHandler extends AbstractSearchIterative
 
     public IterativeScriptExecutionTaskHandler() {
         super("Execute script", OperationConstants.EXECUTE_SCRIPT);
-        setLogFinishInfo(true);     // todo
+        setPreserveStatistics(false);
+        setLogFinishInfo(true); // todo
     }
 
     @PostConstruct