Fix jni WriteBatchThreadedTest

Summary:
WriteBatchThreadedTest is failing, at least on Mac. The problem seems to be `wb` is getting GC before we finish write. Explicitly close it seems to fix it.
Closes https://github.com/facebook/rocksdb/pull/2482

Differential Revision: D5307379

Pulled By: yiwu-arbug

fbshipit-source-id: 8ff7f8170451078c941951f5aafae83afffb7933

java/src/test/java/org/rocksdb/WriteBatchThreadedTest.java

@@ -59,13 +59,14 @@ public class WriteBatchThreadedTest {
       callables.add(new Callable<Void>() {
         @Override
         public Void call() throws RocksDBException {
-          final WriteBatch wb = new WriteBatch();
-          for (int i = offset; i < offset + 100; i++) {
-            wb.put(ByteBuffer.allocate(4).putInt(i).array(),
-                "parallel rocks test".getBytes());
+          try (final WriteBatch wb = new WriteBatch();
+               final WriteOptions w_opt = new WriteOptions()) {
+            for (int i = offset; i < offset + 100; i++) {
+              wb.put(ByteBuffer.allocate(4).putInt(i).array(),
+                  "parallel rocks test".getBytes());
+            }
+            db.write(w_opt, wb);
           }
-          db.write(new WriteOptions(), wb);
-
           return null;
         }
       });