[spotbugs] Don't use class from java.util.concurrent for locking

Use a dedicated Lock object to lock the scheduler in
RepositoryCache#configureEviction to fix spotbugs warning
JLM_JSR166_UTILCONCURRENT_MONITORENTER.

Change-Id: I003dcf0ed1a0a3f4eea5d8a2f51a07473d28a928
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/lib/RepositoryCache.java

@@ -201,6 +201,8 @@ public class RepositoryCache {
 
 	private volatile long expireAfter;
 
+	private Object schedulerLock = new Lock();
+
 	private RepositoryCache() {
 		cacheMap = new ConcurrentHashMap<>();
 		openLocks = new Lock[4];
@@ -214,7 +216,7 @@ public class RepositoryCache {
 			RepositoryCacheConfig repositoryCacheConfig) {
 		expireAfter = repositoryCacheConfig.getExpireAfter();
 		ScheduledThreadPoolExecutor scheduler = WorkQueue.getExecutor();
-		synchronized (scheduler) {
+		synchronized (schedulerLock) {
 			if (cleanupTask != null) {
 				cleanupTask.cancel(false);
 			}