Ideally in Serverless we would like to shard on hash(_id) and use a RecordId format of (hash(_id), per-hash-counter), where the hash function has low cardinality. This allows split cleanup to be on ranges of RecordId's and merge to "stitch" the record heaps

But, we might have to choose a very low cardinality to maintain few write heads per shard, and therefore have to refine/reshard most customers to allow them to have more shards.

This ticket is to do initial tests of write heads vs. insert performance to determine an order of magnitude for a feasible default cardinality.