EXECUTE __insert needs to pass back scas

To support being able to safely do multiple KV operations on the same document with optimistic concurrency, I need to have query's virtual CAS.  This virtual CAS is returned on updates and removes, but not on inserts.

E.g. I need to be able to do:

BEGIN WORK

EXECUTE __insert "txdata":{"kv":true}  // returns "scas"="42"

EXECUTE __update "txdata":{"kv":true,"scas":"42"}