Fix SpotBugs JLM_JSR166_UTILCONCURRENT_MONITORENTER in BulkScanWorker.cleanup()

Replace synchronization on AtomicBoolean with dedicated lock object
to address SpotBugs warning about synchronizing on util.concurrent
instances.

Author: ic0ns

src/main/java/de/rub/nds/crawler/core/BulkScanWorker.java

@@ -82,7 +82,7 @@ public final boolean cleanup() {
         // synchronize such that init and cleanup do not run simultaneously
         // but only synchronize if already initialized
         if (initialized.get()) {
-            synchronized (initialized) {
+            synchronized (initializationLock) {
                 if (activeJobs.get() > 0) {
                     shouldCleanupSelf.set(true);
                     LOGGER.warn(