[Java] Generalize dis-own native handle and refine dispose framework.

Summary:
1. Move disOwnNativeHandle() function from RocksDB to RocksObject
to allow other RocksObject to use disOwnNativeHandle() when its
ownership of native handle has been transferred.

2. RocksObject now has an abstract implementation of dispose(),
which does the following two things.  First, it checks whether
both isOwningNativeHandle() and isInitialized() return true.
If so, it will call the protected abstract function dispose0(),
which all the subclasses of RocksObject should implement.  Second,
it sets nativeHandle_ = 0.  This redesign ensure all subclasses
of RocksObject have the same dispose behavior.

3. All subclasses of RocksObject now should implement dispose0()
instead of dispose(), and dispose0() will be called only when
isInitialized() returns true.

Test Plan:
make rocksdbjava
make jtest

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

Reviewed By: haobo

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D18801

java/org/rocksdb/RocksDB.java

@@ -114,11 +114,9 @@ public class RocksDB extends RocksObject {
     return db;
   }
 
-  @Override public synchronized void dispose() {
-    if (isInitialized()) {
-      dispose(nativeHandle_);
-      nativeHandle_ = 0;
-    }
+  @Override protected void disposeInternal() {
+    assert(isInitialized());
+    disposeInternal(nativeHandle_);
   }
 
   /**
@@ -315,10 +313,6 @@ public class RocksDB extends RocksObject {
     return new RocksIterator(iterator0(nativeHandle_));
   }
 
-  @Override protected void finalize() {
-    close();
-  }
-
   /**
    * Private constructor.
    */
@@ -337,18 +331,6 @@ public class RocksDB extends RocksObject {
     opt.filter_ = null;
   }
 
-  /**
-   * Revoke ownership of the native object.
-   *
-   * This will prevent the object from attempting to delete the underlying
-   * native object in its finalizer. This must be used when another object
-   * (e.g. BackupableDB) takes over ownership of the native object or both
-   * will attempt to delete the underlying object when garbage collected.
-   */
-  protected void disOwnNativeObject() {
-    nativeHandle_ = 0;
-  }
-
   // native methods
   protected native void open(
       long optionsHandle, long cacheSize, String path) throws RocksDBException;
@@ -382,7 +364,7 @@ public class RocksDB extends RocksObject {
       long handle, long writeOptHandle,
       byte[] key, int keyLen) throws RocksDBException;
   protected native long iterator0(long optHandle);
-  protected native void dispose(long handle);
+  private native void disposeInternal(long handle);
 
   protected Filter filter_;
 }