[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/Options.java

@@ -12,7 +12,7 @@ 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 Options {
+public class Options extends RocksObject {
   static final long DEFAULT_CACHE_SIZE = 8 << 20;
   /**
    * Construct options for opening a RocksDB.
@@ -21,7 +21,7 @@ public class Options {
    * an rocksdb::Options in the c++ side.
    */
   public Options() {
-    nativeHandle_ = 0;
+    super();
     cacheSize_ = DEFAULT_CACHE_SIZE;
     newOptions();
   }
@@ -2311,20 +2311,12 @@ public class Options {
    * Release the memory allocated for the current instance
    * in the c++ side.
    */
-  public synchronized void dispose() {
+  @Override public synchronized void dispose() {
     if (isInitialized()) {
       dispose0();
     }
   }
 
-  @Override protected void finalize() {
-    dispose();
-  }
-
-  private boolean isInitialized() {
-    return (nativeHandle_ != 0);
-  }
-
   static final int DEFAULT_PLAIN_TABLE_BLOOM_BITS_PER_KEY = 10;
   static final double DEFAULT_PLAIN_TABLE_HASH_TABLE_RATIO = 0.75;
   static final int DEFAULT_PLAIN_TABLE_INDEX_SPARSENESS = 16;
@@ -2358,7 +2350,6 @@ public class Options {
   private native void useFixedLengthPrefixExtractor(
       long handle, int prefixLength);
 
-  long nativeHandle_;
   long cacheSize_;
   Filter filter_;
 }