[Java] Enable compression_ratio option in DbBenchmark.java

Summary:
Enable the random values in Java DB Bench to be generated based
on the compression_ratio specified in the command-line arguments.

Test Plan:
make rocksdbjava
java/jdb_bench.sh

Reviewers: sdong, ankgup87, haobo

Reviewed By: haobo

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19101