Add support for COMMAND GETKEYS about lettuce

[mp911de]

Removed targeted milestone. This command suggests internal use. Perhaps using it for clustering. lettuce maintains in its own data structures keys and values for commands. Re-use for all commands would employ some sort of different interface to specify different return types for the commands. In contrast to that, the command results could have an additional method to request the keys:

RedisFuture<...> future = connection.set(key, value);
RedisFuture<List<Key>> keys = connection.getKeys(future);

A usage for cluster purposes would require always two commands and an additional sync before firing the real command. The better candidate for cluster control would be the COMMAND command (command details)

[mp911de]

Closed for now, no progress and no demand