Dataset upper bound should be non-optional, enforced

[smklein]

Datasets within Omicron have a field dedicated to their "Maximum provisioned size":

omicron/common/src/sql/dbinit.sql

Lines 172 to 174 in 48285ce

	/* An upper bound on the amount of space that might be in-use */
	size_used INT
	);

This field is:

• Optional
• Not enforced by ZFS quotas/reservations: https://illumos.org/books/zfs-admin/gavwq.html#gazvb

It currently exists as an approximation of crucible dataset size for allocation purposes, but it limited from the perspective of enforcing disk space availability.

We should be implementing / enforcing this for all dataset types. #1231 starts attempting to enforce out-of-space errors, but is incomplete without this cooperation across dataset types.