Decrease Crucible resources in a CTE (#4290)

`decrease_crucible_resource_count_and_soft_delete_volume` is currently
implemented as a rather heavy weight transaction. This commit changes
that to a CTE.

What enabled this was learning that Cockroach can emit JSON through some
built-in functions: part of what made the transaction heavy weight was
that it was serializing the Crucible resources to clean up and recording
that in a column for the soft-deleted Volume. The fact that this can
occur in Cockroach means that the entire transaction can be converted
into a CTE.