Fixed Findbugs issues

- BackupableDB missing call to super.finalize(major)
- WriteBatchTest inefficient String usage(minor)
- RocksDB local dead variable store(medium)

java/org/rocksdb/BackupableDB.java

@@ -82,6 +82,7 @@ public class BackupableDB extends RocksDB {
 
   @Override protected void finalize() {
     close();
+    super.finalize();
   }
 
   protected native void open(long rocksDBHandle, long backupDBOptionsHandle);

java/org/rocksdb/RocksDB.java

@@ -103,8 +103,6 @@ public class RocksDB extends RocksObject {
    * @see Options.createIfMissing()
    */
   public static RocksDB open(String path) throws RocksDBException {
-    RocksDB db = new RocksDB();
-
     // This allows to use the rocksjni default Options instead of
     // the c++ one.
     Options options = new Options();

java/org/rocksdb/WriteBatchTest.java

@@ -53,7 +53,7 @@ public class WriteBatchTest {
       WriteBatchInternal.setSequence(batch, 100);
       assert(100 == WriteBatchInternal.sequence(batch));
       assert(3 == batch.count());
-      assert(new String("Put(baz, boo)@102" +
+      assert(("Put(baz, boo)@102" +
               "Delete(box)@101" +
               "Put(foo, bar)@100")
                 .equals(new String(getContents(batch), "US-ASCII")));
@@ -79,13 +79,13 @@ public class WriteBatchTest {
       b2.clear();
       b2.put("b".getBytes("US-ASCII"), "vb".getBytes("US-ASCII"));
       WriteBatchInternal.append(b1, b2);
-      assert(new String("Put(a, va)@200" +
+      assert(("Put(a, va)@200" +
               "Put(b, vb)@201")
                 .equals(new String(getContents(b1), "US-ASCII")));
       assert(2 == b1.count());
       b2.remove("foo".getBytes("US-ASCII"));
       WriteBatchInternal.append(b1, b2);
-      assert(new String("Put(a, va)@200" +
+      assert(("Put(a, va)@200" +
               "Put(b, vb)@202" +
               "Put(b, vb)@201" +
               "Delete(foo)@203")
@@ -108,7 +108,7 @@ public class WriteBatchTest {
       batch.putLogData("blob2".getBytes("US-ASCII"));
       batch.merge("foo".getBytes("US-ASCII"), "bar".getBytes("US-ASCII"));
       assert(5 == batch.count());
-      assert(new String("Merge(foo, bar)@4" +
+      assert(("Merge(foo, bar)@4" +
               "Put(k1, v1)@0" +
               "Delete(k2)@3" +
               "Put(k2, v2)@1" +