Separate channel and channel creator in key path in 04-channel v2

[gjermundgaraba]

In 04-channel V2, channel creators are currently keyed by channel_key{channel_id}/creator_key. This means that if you iterate over channels by channel_key, the iterator will return both channels and creators.

To avoid this, we should change the key structure for channels creators to be prefixed with the creator_key instead. For instance creator_key/{channel_id}.

[DimitrisJim]

done in #7696