[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/RocksIterator.java

@@ -118,15 +118,13 @@ public class RocksIterator extends RocksObject {
   /**
    * Deletes underlying C++ iterator pointer.
    */
-  @Override public synchronized void dispose() {
-    if(isInitialized()) {
-      dispose(nativeHandle_);
-      nativeHandle_ = 0;
-    }
+  @Override protected void disposeInternal() {
+    assert(isInitialized());
+    disposeInternal(nativeHandle_);
   }
 
   private native boolean isValid0(long handle);
-  private native void dispose(long handle);
+  private native void disposeInternal(long handle);
   private native void seekToFirst0(long handle);
   private native void seekToLast0(long handle);
   private native void next0(long handle);