[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/ReadOptions.java

@@ -11,9 +11,9 @@ package org.rocksdb;
  * Note that dispose() must be called before an Options instance
  * become out-of-scope to release the allocated memory in c++.
  */
-public class ReadOptions {
+public class ReadOptions extends RocksObject {
   public ReadOptions() {
-    nativeHandle_ = 0;
+    super();
     newReadOptions();
   }
   private native void newReadOptions();
@@ -24,7 +24,7 @@ public class ReadOptions {
    *
    * Calling other methods after dispose() leads to undefined behavior.
    */
-  public synchronized void dispose() {
+  @Override public synchronized void dispose() {
     if (isInitialized()) {
       dispose(nativeHandle_);
     }
@@ -127,10 +127,4 @@ public class ReadOptions {
   }
   private native void setTailing(
       long handle, boolean tailing);
-
-  protected long nativeHandle_;
-
-  private boolean isInitialized() {
-    return nativeHandle_ != 0;
-  }
 }