Use datastore segment index for ad-hoc meshing

[fm3]

For on-disk segmentation layers (no annotation layers), the ad-hoc meshing could use the same optimization as the one for volume annotation layers if there is a segment index file.

Needs

• Datastore route to expose the available positions from the segment index (same format as tracingstore has) – Maybe for datastore-tracingstore communication that could then also be used, in place of current GET /datasets/:organizationName/:dataSetName/layers/:dataLayerName/segmentIndex/:segmentId
• Frontend to use that

Should also be used by the fullMesh.stl route