Crucible volumes are created by layering read-write regions over a
hierarchy of read-only resources. Originally only a region snapshot
could be used as a read-only resource for a volume. With the
introduction of read-only regions (created during the region snapshot
replacement process) this is no longer true!

Read-only resources can be used by many volumes, and because of this
they need to have a reference count so they can be deleted when they're
not referenced anymore. The region_snapshot table uses a
`volume_references` column, which counts how many uses there are. The
region table does not have this column, and more over a simple integer
works for reference counting but does not tell you _what_ volume that
use is from. This can be determined (see omdb's validate volume
references command) but it's information that is tossed out, as Nexus
knows what volumes use what resources! Instead, record what read-only
resources a volume uses in a new table.

As part of the schema change to add the new `volume_resource_usage`
table, a migration is included that will create the appropriate records
for all region snapshots.

In testing, a few bugs were found: the worst being that read-only
regions did not have their read_only column set to true. This would be a
problem if read-only regions are created, but they're currently only
created during region snapshot replacement. To detect if any of these
regions were created, find all regions that were allocated for a
snapshot volume:

    SELECT id FROM region
    WHERE volume_id IN (SELECT volume_id FROM snapshot);

A similar bug was found in the simulated Crucible agent.

This commit also reverts oxidecomputer#6728, enabling region snapshot replacement
again - it was disabled due to a lack of read-only region reference
counting, so it can be enabled once again.

The garbage collection of read/write regions must be separate from
read-only regions:

- read/write regions are garbage collected by either being deleted
  during a volume soft delete, or by appearing later during the "find
  deleted volume regions" section of the volume delete saga

- read-only regions are garbage collected only in the volume soft delete
  code, when there are no more references to them

`find_deleted_volume_regions` was changed to only operate on read/write
regions, and no longer returns the optional RegionSnapshot object: that
check was moved from the volume delete saga into the function, as it
didn't make sense that it was separated.

This commit also adds checks to validate that invariants related to
volumes are not violated during tests. One invalid test was deleted
(regions will never be deleted when they're in use!)

In order to properly test the separate region deletion routines, the
first part of the fixes for dealing with deleted volumes during region
snapshot replacement were brought in from that branch: these are the
changes to region_snapshot_replacement_step.rs and
region_snapshot_replacement_start.rs.

add comment to table as well