labelmap POST /blocks behavior w.r.t. the label index

[stuarteberg]

(Summarizing a brief in-person discussion.)

We need to review the behavior of POST /blocks and POST /raw for labelmap instances with respect to the label index and the kafka log.

1. In general, do we intend to support those endpoints for labelmap instances (beyond the initial ingestion phase)? Currently, POST /blocks does NOT update the label index appropriately, and neither does POST /raw.

2. If so, how should we track supervoxel/body provenance in such cases? Without the above endpoints, supervoxels can only appear/disappear via /split or /split-supervoxel, which are logged to kafka. Similarly, bodies can only appear via /cleave and disappear via /merge. But if we permit the above POST endpoints, then supervoxels and bodies can disappear without a trace in the kafka log. We should define new kafka messages to handle these cases.