[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/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_;