Add Scylla to benchmark

[arothuis]

Scylla is intended to be a more performant, drop-in replacement for Cassandra.

It has been benchmarked a bunch, but it would be interesting to see in this benchmark as well. The scripts for Cassandra code can be copied and modified slightly to fit Scylla.

[LeeHampton]

Agreed @arothuis -- I wanted to add Scylla (maybe KairosDB as well) when I was doing Cassandra benchmarks but didn't have time. Depending on whether the Scylla wire protocol and/or data model has significant differences from Cassandra, we may actually be able to just extend the existing Cassandra packages with a Scylla option (e.g., "engine=scylla" or something).

In theory the data/query generation packages shouldn't need to be touched, but I'm not 100 % sure there are no differences in the way data is loaded/queried.