Cannot un-delete a disk with the same name

[jmpesp]

Disks for which their delete saga unwound prior to the fixes in #4547 would be "phantom" disks: they would be soft-deleted but would consume virtual resources and count towards quotas. If the user created a disk with the same name after attempting to delete the original disk, this new disk would prevent the phantom disk from being un-deleted due to the constraint that disk names within a project must be unique:

CREATE UNIQUE INDEX IF NOT EXISTS lookup_disk_by_project ON omicron.public.disk (
    project_id,
    name
) WHERE
    time_deleted IS NULL;

As written today, the background task does not rename the disk, so the un-delete step would fail every time the background task is triggered.

Possible fixes:

• rename the disk to something unique during un-deletion
• trigger the disk delete saga instead of un-deleting the disk