perf: use case-insensitive comparison instead of normalization for command names

[supercaracal]

RedisClient::Cluster::NormalizedCmdName is too slow. String#casecmp would be faster and better. Also, redundant type castings should be removed.

A lowercase command is the best performance in our gem.

• get foo
  • No additional memory allocations.
  • No additional lookups to a hash object.
• GET foo
  • String#downcase method is called to refer the command information.
    • https://redis.io/docs/latest/commands/command/
    • like: @command['get']