Cassandra DB driver

This implementation is geared towards yugabyte.

Migration from v0.4.0 to v0.5.0

The addition of row_id as a TIMEUUID as a simplistic version of state hash. Since row_id can be null, gungnir will work with both database schemas. In order to do long polling, gungnir db driver will need to be updated. Svalinn is not backwards compatible as the insert statement has changed to include the TIMEUUID.

The following is the migration script from v0.4.0 to v0.5.0

ALTER TABLE devices.events ADD row_id TIMEUUID;
CREATE INDEX search_by_row_id ON devices.events
    (device_id, row_id) 
    WITH CLUSTERING ORDER BY (row_id DESC)
    AND default_time_to_live = 2768400
    AND transactions = {'enabled': 'false', 'consistency_level':'user_enforced'};