Cassandra backend testing

Cassandra backend is based on [DataStax Cassandra Java driver](https://github.com/datastax/java-driver).

Cassandra driver not provides Cassandra mock instance to run tests. It is possible to use SCassandra and CassandraUnit to launch an embedded Cassandra, but this way is expensive by machine resources, moreover it is not the fastest way to run test suit. A real Cassandra instance limited by memory (exmaple: in a Docker container) can be used as a solution, that would be fastest and cheapest way to launch tests. Before running Cassandra tests, be sure, that a local (127.0.0.1) Cassandra instance is available. Script to start local Cassandra instance using Docker is provided here

One can also use ccm for running a Cassandra cluster on localhost for testing purposes. ccm helps to manage and join nodes into a local cluster and can help in stress testing against a more real-world cluster environment.

### Mac OS X / Windows users

Docker is not supported by Mac OS X / Windows natively, it is possible to use Docker for Mac / Windows or smth else. In case of a Docker Machine, it is important to forward necessary ports, in our case to forward Cassandra cql native transport port 9042 on localhost, from docker container to a localhost, from oracle vm where docker container is started. It can be done using the following command:

vboxmanage controlvm dev natpf1 "9042,tcp,,9042,,9042"

After that Cassandra would be available at localhost:9042.