Measure + Improve server-side performance for supervoxel proofreading

[fm3]

a.k.a. editable mappings.

Also compare #6288

Ideas:

• [ ] Optimize Roundtrip?
• [ ] Use low-level typed data processing (no UnsignedInteger)
• [ ] Is the size limit for agglomerate skeletons too tight?
• [x] save editable mapping in distributed keys in fossildb (agglomerateToGraph)

[philippotto]

While the supervoxel proofreading performance has been improved by #6909, there is still some room for improvement. We should investigate the current bottlenecks for proofreadingmedium-sized agglomerate files (splits and merge oeprations). Also, the issue description above gives some ideas about what could be done.