SLING-7811 - NPE when repository is starting up

Make sure that each repository startup thread has an individual name.
This makes it easier to distinguish repository restart issues during
startup.
diff --git a/src/main/java/org/apache/sling/jcr/base/AbstractSlingRepositoryManager.java b/src/main/java/org/apache/sling/jcr/base/AbstractSlingRepositoryManager.java
index bedb9e7..002d8bc 100644
--- a/src/main/java/org/apache/sling/jcr/base/AbstractSlingRepositoryManager.java
+++ b/src/main/java/org/apache/sling/jcr/base/AbstractSlingRepositoryManager.java
@@ -85,6 +85,8 @@
 @ProviderType
 public abstract class AbstractSlingRepositoryManager {
 
+    private static final AtomicInteger startupCounter = new AtomicInteger();
+
     /** default log */
     private final Logger log = LoggerFactory.getLogger(getClass());
 
@@ -452,7 +454,7 @@
         // start repository asynchronously to allow LoginAdminWhitelist to become available
         // NOTE: making this conditional allows tests to register a mock whitelist before
         // activating the RepositoryManager, so they don't need to deal with async startup
-        startupThread = new Thread("Apache Sling Repository Startup Thread") {
+        startupThread = new Thread("Apache Sling Repository Startup Thread #" + startupCounter.incrementAndGet()) {
             @Override
             public void run() {
                 try {