DATAREDIS-290 - Add support for SCAN command.

`SCAN` is currently only supported by `jedis` and can be emulated for `lettuce` via eval.
`Srp` and `JRedis` will respond with `UnsupportedOperationException`.

We provide a `Cursor` implementation allowing to scroll through the responses provided by the underlying connection. The cursor will fetch additional results from redis server whenever its starting point has not been reached and there are no more already loaded items available.