[CELEBORN-1516] DynamicConfigServiceFactory should support singleton

### What changes were proposed in this pull request?
DynamicConfigServiceFactory supports singleton.

### Why are the changes needed?
Improve code.

### Does this PR introduce _any_ user-facing change?
No.

### How was this patch tested?
Existing UTs.

Closes #2635 from leixm/singleton.

Authored-by: Xianming Lei <[email protected]>
Signed-off-by: Shuang <[email protected]>
(cherry picked from commit d5b124d)
Signed-off-by: Shuang <[email protected]>

service/src/main/java/org/apache/celeborn/server/common/service/config/DynamicConfigServiceFactory.java

@@ -22,18 +22,26 @@
 import org.apache.celeborn.common.CelebornConf;
 
 public class DynamicConfigServiceFactory {
+  private static volatile ConfigService _INSTANCE;
 
   public static ConfigService getConfigService(CelebornConf celebornConf) throws IOException {
     if (celebornConf.dynamicConfigStoreBackend().isEmpty()) {
       return null;
-    } else {
-      String configStoreBackend = celebornConf.dynamicConfigStoreBackend().get();
-      // celeborn.dynamicConfig.store.backend checks value with FS, DB.
-      if ("FS".equals(configStoreBackend)) {
-        return new FsConfigServiceImpl(celebornConf);
-      } else {
-        return new DbConfigServiceImpl(celebornConf);
+    }
+
+    if (_INSTANCE == null) {
+      synchronized (DynamicConfigServiceFactory.class) {
+        if (_INSTANCE == null) {
+          String configStoreBackend = celebornConf.dynamicConfigStoreBackend().get();
+          if ("FS".equals(configStoreBackend)) {
+            _INSTANCE = new FsConfigServiceImpl(celebornConf);
+          } else {
+            _INSTANCE = new DbConfigServiceImpl(celebornConf);
+          }
+        }
       }
     }
+
+    return _INSTANCE;
   }
 }