Wrong codec when using multiple codec with in-memory kv store.

When creating multiple client with different codec, only the first registered codec will be used when using the in-memory KV.

This has caused me multiple issues while trying to use another in-memory store in loki.

see https://github.com/grafana/loki/blob/main/vendor/github.com/grafana/dskit/kv/client.go#L140-L142