[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() {

java/org/rocksdb/RocksIterator.java

@@ -11,13 +11,13 @@ package org.rocksdb;
  * are provided by this library.  In particular, iterators are provided
  * to access the contents of a Table or a DB.
  *
- * Multiple threads can invoke const methods on an Iterator without
+ * Multiple threads can invoke const methods on an RocksIterator without
  * external synchronization, but if any of the threads may call a
- * non-const method, all threads accessing the same Iterator must use
+ * non-const method, all threads accessing the same RocksIterator must use
  * external synchronization.
  */
-public class Iterator extends RocksObject {
-  public Iterator(long nativeHandle) {
+public class RocksIterator extends RocksObject {
+  public RocksIterator(long nativeHandle) {
     super();
     nativeHandle_ = nativeHandle;
   }

java/org/rocksdb/benchmark/DbBenchmark.java

@@ -416,7 +416,7 @@ public class DbBenchmark {
       super(tid, randSeed, numEntries, keyRange);
     }
     @Override public void runTask() throws RocksDBException {
-      org.rocksdb.Iterator iter = db_.newIterator();
+      RocksIterator iter = db_.newIterator();
       long i;
       for (iter.seekToFirst(), i = 0;
            iter.isValid() && i < numEntries_;