For various admin and auditing purposes, it would be nice if there were a way to quickly retrieve certain values:

• The set of supervoxels in a body (without the need for dvid to read the entire labelindex)
• The size of any supervoxel, including retired supervoxels. (Right now, the /labelsz endpoint can quickly return body sizes, but not supervoxel sizes.)
• The complete list of ALL body IDs in the volume
• The complete list of ALL supervoxel IDs in the volume.

Related issues: #297, #323