S3 object naming

[SimonUnge]

Is your feature request related to a problem? Please describe.

For object keys, we shoud use hash prefixes to distribute load across S3 partitions.

${BUCKETNAME}/
├── {hash(stream-name)}/streams/{vhost}/{stream-name}/ %% Key structure
│   ├── segments/{segment-id}/
│   │   └── ...

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