Add MEMORY USAGE command as an alias to existing DISK USAGE command

[torwig]

Since Redis has the MEMORY command, tools like RedisInsight use it to show information to the user.
Currently, RedisInsight can't show memory usage by keys because Kvrocks doesn't support the MEMORY USAGE command.
Solution: make the MEMORY USAGE command an alias to the DISK USAGE command.
Currently, the MEMORY USAGE command doesn't support the optional SAMPLES option.
The integration test basically checks that Kvrocks support the new command and the response is an integer value conforming to the Redis spec.

[aleksraiden]

Good, thanks! Now I use a redisInsight tool im work and we have two separated service, redis and kvroks, and this features very good to me!

[git-hulk]

Thanks all, merging...