[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/rocksjni/iterator.cc

@@ -26,7 +26,7 @@ jboolean Java_org_rocksdb_Iterator_isValid0(
 
 /*
  * Class:     org_rocksdb_Iterator
- * Method:    close0
+ * Method:    seekToFirst0
  * Signature: (J)V
  */
 void Java_org_rocksdb_Iterator_seekToFirst0(
@@ -135,13 +135,11 @@ void Java_org_rocksdb_Iterator_status0(
 
 /*
  * Class:     org_rocksdb_Iterator
- * Method:    status0
+ * Method:    dispose
  * Signature: (J)V
  */
-void Java_org_rocksdb_Iterator_close0(
+void Java_org_rocksdb_Iterator_dispose(
     JNIEnv* env, jobject jobj, jlong handle) {
   auto it = reinterpret_cast<rocksdb::Iterator*>(handle);
   delete it;
-
-  rocksdb::IteratorJni::setHandle(env, jobj, nullptr);
 }

java/rocksjni/rocksjni.cc

@@ -415,16 +415,14 @@ void Java_org_rocksdb_RocksDB_remove__JJ_3BI(
 
 /*
  * Class:     org_rocksdb_RocksDB
- * Method:    close0
- * Signature: ()V
+ * Method:    dispose
+ * Signature: (J)V
  */
-void Java_org_rocksdb_RocksDB_close0(
-    JNIEnv* env, jobject java_db) {
-  rocksdb::DB* db = rocksdb::RocksDBJni::getHandle(env, java_db);
+void Java_org_rocksdb_RocksDB_dispose(
+    JNIEnv* env, jobject java_db, jlong jhandle) {
+  auto db = reinterpret_cast<rocksdb::DB*>(jhandle);
   assert(db != nullptr);
   delete db;
-
-  rocksdb::RocksDBJni::setHandle(env, java_db, nullptr);
 }
 
 /*