Add benchmarks for Cassandra

Summary:
python runbenchmarks.py all
> [cassandra]: running benchmark...
> [cassandra]: insert_throughput
  * 1 thread(s).
  - chunk_size=500 took 19.0937168598s
  - chunk_size=1000 took 22.4170408249s
  - chunk_size=5000 took 17.8571779728s
  - chunk_size=10000 took 17.5737910271s
  * 5 thread(s).
  - chunk_size=500 took 17.5144238472s
  - chunk_size=1000 took 16.564604044s
  - chunk_size=5000 took 16.472630024s
  - chunk_size=10000 took 16.652575016s
  * 10 thread(s).
  - chunk_size=500 took 16.858784914s
  - chunk_size=1000 took 15.9280769825s
  - chunk_size=5000 took 16.1751189232s
  - chunk_size=10000 took 16.6325390339s
> [cassandra]: num_per_bucket
  - n=200000 took 0.370865821838s
  - n=400000 took 0.396054029465s
  - n=600000 took 0.453202009201s
  - n=800000 took 0.441138982773s
  - n=1000000 took 0.411331176758s
  - n=1200000 took 0.456668138504s
  - n=1400000 took 0.41574883461s
  - n=1600000 took 0.519033193588s
  - n=1800000 took 0.461040019989s
  - n=2000000 took 0.405261993408s
> [cassandra]: shards_per_bucket
  - shards=1 took 5.87006187439s
  - shards=3 took 6.65000104904s
  - shards=6 took 7.32550001144s
  - shards=12 took 8.61449790001s
  - shards=24 took 9.56229281425s
> [cassandra]: cleaning up...
> [cassandra]: took 772.540820122s

Test Plan: -

Reviewers: marcua

Reviewed By: marcua

Differential Revision: https://phabricator.locu.com/D7169