Consider flag for order-agnostic payload

[agavra]

After #27 (see #26 for motivation) we now deterministically serialize the payload when we merge records together. This ensures that two records with the same fields are always serialized in the same way so byte-array comparison will return whether or not they are equal without deserializing the records.

Some use cases may not necessitate byte-to-byte equality between records in which case they can benefit from order-agnostic serialization. This makes the merge operation about 20% faster, but it adds complexity and a serious "foot-gun" to the project.

Not sure the tradeoff is worth it, but I'm opening the issue for discussion.