Fix SpotBugs JLM_JSR166_UTILCONCURRENT_MONITORENTER in BulkScanWorker.init()

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

@@ -24,6 +24,7 @@ public abstract class BulkScanWorker<T extends ScanConfig> {
     private final AtomicInteger activeJobs = new AtomicInteger(0);
     private final AtomicBoolean initialized = new AtomicBoolean(false);
     private final AtomicBoolean shouldCleanupSelf = new AtomicBoolean(false);
+    private final Object initializationLock = new Object();
     protected final String bulkScanId;
     protected final T scanConfig;
 
@@ -67,7 +68,7 @@ public final boolean init() {
         // synchronize such that no thread runs before being initialized
         // but only synchronize if not already initialized
         if (!initialized.get()) {
-            synchronized (initialized) {
+            synchronized (initializationLock) {
                 if (!initialized.getAndSet(true)) {
                     initInternal();
                     return true;