[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/BackupableDB.java

@@ -55,8 +55,8 @@ public class BackupableDB extends RocksDB {
    * should be transparent to Java developers.
    */
   @Override public synchronized void close() {
-    if (isOpened()) {
-      super.close0();
+    if (isInitialized()) {
+      super.close();
     }
   }
 
@@ -73,10 +73,6 @@ public class BackupableDB extends RocksDB {
     close();
   }
 
-  private boolean isOpened() {
-    return nativeHandle_ != 0;
-  }
-
   protected native void open(long rocksDBHandle, long backupDBOptionsHandle);
   protected native void createNewBackup(long handle, boolean flag);