[Java] Include WriteBatch into RocksDBSample.java, fix how DbBenchmark.java handles WriteBatch.

Summary:
Include WriteBatch into RocksDBSample.java, fix how DbBenchmark.java handles WriteBatch.
Previously DbBenchmark.java does not use WriteBatch when benchmarks is set to fillbatch.

Test Plan:
make rocksdbjava -j32
make jtest
make jdb_bench
cd java
./jdb_bench.sh --benchmarks=fillbatch

Reviewers: naveenatceg, ljin, sdong, ankgup87

Reviewed By: ankgup87

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D22983