[Java] Add RocksObject, the base class of all java objects with a c++ pointer.

Summary:
Add RocksObject, the base class of all java objects which has a c++ pointer.
While the finalizer of a RocksObject will release its c++ resource, it is
suggested to call its RocksObject.dispose() to manually release its
c++ resource.

Existing RocksDB java classes are now extending RocksObject.

Test Plan:
make rocksdbjava
make jtest
make jdb_bench

Reviewers: haobo, dhruba, sdong, ankgup87, rsumbaly, swapnilghike, zzbennett

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18411

java/org/rocksdb/WriteBatch.java

@@ -24,9 +24,9 @@ import java.util.*;
  * non-const method, all threads accessing the same WriteBatch must use
  * external synchronization.
  */
-public class WriteBatch {
+public class WriteBatch extends RocksObject {
   public WriteBatch() {
-    nativeHandle_ = 0;
+    super();
     newWriteBatch(0);
   }
 
@@ -86,16 +86,12 @@ public class WriteBatch {
   /**
    * Delete the c++ side pointer.
    */
-  public synchronized void dispose() {
-    if (nativeHandle_ != 0) {
+  @Override public synchronized void dispose() {
+    if (isInitialized()) {
       dispose0();
     }
   }
 
-  @Override protected void finalize() {
-    dispose();
-  }
-
   private native void newWriteBatch(int reserved_bytes);
   private native void put(byte[] key, int keyLen,
                           byte[] value, int valueLen);
@@ -104,8 +100,6 @@ public class WriteBatch {
   private native void remove(byte[] key, int keyLen);
   private native void putLogData(byte[] blob, int blobLen);
   private native void dispose0();
-
-  long nativeHandle_;
 }
 
 /**