Make EventData building based on IEvent synchronization independent from factory instance

Using factory instance for blocking is prone to sync issues. We don't document
possibility to block on IEventDataFactory instance anyway.

DEVSIX-5515

Author: yulian-gaponenko

kernel/src/main/java/com/itextpdf/kernel/counter/data/EventDataHandler.java

@@ -71,6 +71,7 @@ This file is part of the iText (R) project.
  * @param <V> data type
  */
 public abstract class EventDataHandler<T, V extends EventData<T>> {
+    private final Object createLock = new Object();
     private final Object processLock = new Object();
 
     private final IEventDataCache<T, V> cache;
@@ -97,8 +98,7 @@ public List<V> clear() {
 
     public void register(IEvent event, IMetaInfo metaInfo) {
         V data;
-        //Synchronization is left here mostly in consistency with cache and process, but factories are usually not thread safe anyway.
-        synchronized (factory) {
+        synchronized (createLock) {
             data = factory.create(event, metaInfo);
         }
         if (data != null) {