Make NML import more performant and resilient

[daniel-wer]

• Better performance, by keeping trees and groups mutable until put into the store (From ~5s to ~1.5s for this part for a 200,000 node tracing)
• Allow missing tree names and assign a new tree name for missing and empty tree names
• Align tree timestamp assignment to backend NML parser. Replace invalid timestamps with a default timestamp of 0.
• Fixed that validation errors are shown for NML-like files
• Split unconnected trees during NML import and wrap them in a group.

Ideally, I'd like to avoid the redefinition of the MutableTree type and, therefore, experimented with recursive read-only instructions for flow, but I couldn't get those to work for the diffable maps :/
(for reference, the most success I had was with the solution outlined in facebook/flow#5844 (comment))

URL of deployed dev instance (used for testing):

• https://___.webknossos.xyz

Steps to test:

• knossos_nmls.zip
• Import these knossos NMLs in the tracing view.
  • Unconnected trees should be split
  • There should be no errors during saving
• Try to import an NML where the trees have no name or the nodes have no attributes (except for position and id)
• Check that validation errors are shown for NML-like files, e.g. by removing the id attribute of one of the nodes before upload.

Issues:

• contributes to Make import of NMLs more resilient #4374 (needs to be done in backend as well)
• fixes Frontend NML parser: show error details in toast #4548

---

• Updated changelog
• Updated migration guide if applicable
• Updated documentation if applicable
• Adapted wk-connect if datastore API changes
• Needs datastore update after deployment
• Ready for review

[philippotto]

Top notch work 🎉 I only left some minor performance ideas which should be easy to integrate.

/edit: Also tested the provided NMLs and everything worked fine 👍

[daniel-wer]

Thanks for the review, I updated flow to a somewhat newer version, because the flatMap type definition was missing - fixed the new errors as good as possible. The newest flow version adds lots of new errors, so that'll be a project for another day ^^

[daniel-wer]

@philippotto Could you have a look at my new changes and maybe test whether yarn flow test works for you locally with this version? :)

[philippotto]

@philippotto Could you have a look at my new changes and maybe test whether yarn flow test works for you locally with this version? :)

After I upgraded node to v12, everything worked fine :) Maybe post a PSA in slack after this is merged?

[daniel-wer]

Absolutely, will do, thanks for testing :)