record: ensure that frames are stored flat (and not in bucket) in the record file

[netgusto]

No description provided.

[netgusto]

The frames should not be stored in buckets in the record file, so that replay tps may be chosen independently from record tps (particularly useful to replay sessions recorded using high frequency tps during training).

Then the original tps has to be kept in the metadata related to the record, to be able to replay at original speed if need be.