Kill RDB child process on CLUSTER RESET

```
I think it is a coincidence that we don't use chained replication in the cluster mode but that is an orthogonal decision so it is still preferable imo to not take that dependency.

Yeah I think cluster reset should kill the rdb child process too.
```

_Originally posted by @PingXie in https://github.com/valkey-io/valkey/pull/1674#discussion_r2031909790_
            