DATAREDIS-756 - Order results of partitioned multi-key commands by positional keys.

We now order and assemble results of multi-key commands (e.g. MGET with cross-slot keys) that are executed on different nodes by positional keys to retain duplicate keys in the requested order and retain the server response.

Previously duplicate keys were treated as set of keys and the response didn't match the requested keys.

Original Pull Request: #303

Author: mp911de