YARN-4956. findbug issue on LevelDBCacheTimelineStore. (Zhiyuan Yang via gtcarrera9)

(cherry picked from commit f16722d2ef31338a57a13e2c8d18c1c62d58bbaf)

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-timeline-pluginstorage/src/main/java/org/apache/hadoop/yarn/server/timeline/LevelDBCacheTimelineStore.java

@@ -78,7 +78,7 @@ public class LevelDBCacheTimelineStore extends KeyValueBasedTimelineStore {
   }
 
   @Override
-  protected void serviceInit(Configuration conf) throws Exception {
+  protected synchronized void serviceInit(Configuration conf) throws Exception {
     configuration = conf;
     Options options = new Options();
     options.createIfMissing(true);