Store skeletons as JSON in RocksDB

[jfrohnhofen]

The way skeleton tracings are currently stored in mongoDB does not scale to >200.000 nodes and must thus be stored in a different way.

• move skeleton tracings to datastore
• move skeleton tracing format to JSON
• save skeleton modifications to write-log
• apply skeleton modifications on skeleton requests
• authentication via tokens
• validate skeleton schema (and find/define NML schema) (see https://discuss.webknossos.org/t/backend-validation-of-skeletons/122)
• add support for skeleton versions (see https://discuss.webknossos.org/t/undo-user-actions/146)

Log Time

[jfrohnhofen]

related
https://discuss.webknossos.org/t/backend-validation-of-skeletons/122
https://discuss.webknossos.org/t/performance-issues-with-large-amounts-of-data/86
#51

[daniel-wer]

@fm3 @jfrohnhofen I've made the changes in the frontend to fetch the annotation, dataset and tracing separately (and repaired all places in the frontend that were affected by that). There's still a couple of ToDos, which are being kept track of in the collabedit pad.
At least it's possible to create a skeletontracing now and look at the data. Volume tracings don't seem to work yet and there's a small error for Viewing tracings that I already mentioned to @fm3.

If I understood correctly, the update actions should now be sent to the dataStore instead of the server. Same route or did something change there?

[daniel-wer]

Just for reference, the relevant branch is bravenewworldSkeletons.