YCSB client seems to have debug log enabled

[tianleq]

When looking into allocation stats of cassandra, zixian and I noticed lots of allocation comes from ycsb client's debug logging. The relevant call site is https://github.com/brianfrankcooper/YCSB/blob/dbe92a1fbd24be061fcf01e56838afb83e6f9c58/cassandra/src/main/java/site/ycsb/db/CassandraCQLClient.java#L497