[Java] Rename org.rocksdb.Iterator to org.rocksdb.RocksIterator.

Summary:
Renamed Iterator to RocksIterator to avoid potential confliction with
the Java built-in Iterator.

Test Plan:
make rocksdbjava
make jtest

Reviewers: haobo, dhruba, sdong, ankgup87

Reviewed By: sdong

CC: leveldb, rsumbaly, swapnilghike, zzbennett

Differential Revision: https://reviews.facebook.net/D18615

java/org/rocksdb/RocksDB.java

@@ -311,8 +311,8 @@ public class RocksDB extends RocksObject {
    *
    * @return instance of iterator object.
    */
-  public Iterator newIterator() {
-    return new Iterator(iterator0(nativeHandle_));
+  public RocksIterator newIterator() {
+    return new RocksIterator(iterator0(nativeHandle_));
   }
 
   @Override protected void finalize() {