[nexus] As a long-term goal, stop using "dataset" CockroachDB table

[smklein]

It would be nice if we could rely on inventory for "datasets we observe", and the blueprint for "datasets we want to have".

There exists a dataset table that's kinda redundant with the blueprint information, but exists for historical reasons:

omicron/schema/crdb/dbinit.sql

Lines 559 to 604 in 15b4c45

	/*
	* A dataset of allocated space within a zpool.
	*/
	CREATE TABLE IF NOT EXISTS omicron.public.dataset (
	/* Identity metadata (asset) */
	id UUID PRIMARY KEY,
	time_created TIMESTAMPTZ NOT NULL,
	time_modified TIMESTAMPTZ NOT NULL,
	time_deleted TIMESTAMPTZ,
	rcgen INT NOT NULL,
	/* FK into the Pool table */
	pool_id UUID NOT NULL,
	/* Contact information for the dataset */
	ip INET,
	port INT4 CHECK (port BETWEEN 0 AND 65535),
	kind omicron.public.dataset_kind NOT NULL,
	/* An upper bound on the amount of space that might be in-use */
	size_used INT,
	/* Only valid if kind = zone -- the name of this zone */
	zone_name TEXT,
	quota INT8,
	reservation INT8,
	compression TEXT,
	/* Crucible must make use of 'size_used'; other datasets manage their own storage */
	CONSTRAINT size_used_column_set_for_crucible CHECK (
	(kind != 'crucible') OR
	(kind = 'crucible' AND size_used IS NOT NULL)
	),
	CONSTRAINT ip_and_port_set_for_crucible CHECK (
	(kind != 'crucible') OR
	(kind = 'crucible' AND ip IS NOT NULL and port IS NOT NULL)
	),
	CONSTRAINT zone_name_for_zone_kind CHECK (
	(kind != 'zone') OR
	(kind = 'zone' AND zone_name IS NOT NULL)
	)
	);

We used to have a similar table for services, but got rid of it in favor of blueprints.
It would be nice if we could do the same for datasets!

• Stop relying on omicron.public.dataset records to look up Crucible socket addresses
• Stop relying on omicron.public.dataset records to track Crucible space usage