[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