Motivation:

There are several cases where "command" could refer to a command keyword, or an entire message (keyword + args). This made working with `RedisCommand` and it's documentation ambiguous.

Modifications:

- Rename `RedisCommand.command` to `message`
- Rename initializer labels to `message` and `responsePromise`

Result:

When encountering a `RedisCommand` everyone should know that they are dealing with a message that should be sent to Redis as soon as possible.