Renames `key` to `newkey` if `newkey` does not yet exist. It returns an error
when `key` does not exist.

In Cluster mode, both `key` and `newkey` must be in the same **hash slot**,
meaning that in practice only keys that have the same hash tag can be reliably
renamed in cluster.

@history

- `<= 3.2.0`: Before Redis 3.2.0, an error is returned if source and destination
  names are the same.

@return

@integer-reply, specifically:

- `1` if `key` was renamed to `newkey`.
- `0` if `newkey` already exists.

@examples

```cli
SET mykey "Hello"
SET myotherkey "World"
RENAMENX mykey myotherkey
GET myotherkey
```