Avoid suboptimal bucket loading sequence for initial loading of dataset

[philippotto]

Reuse bucket_picker priorities when requesting missing buckets via pullqueu

Mailable description of changes:

• The order in which initial buckets are loaded when viewing a dataset was streamlined.

URL of deployed dev instance (used for testing):

• https://reusepickerpriorities.webknossos.xyz

Steps to test:

• open tracing in ortho and arbitrary views. ensure that bucket loading order makes sense.
• especially, initial loading should be from center to the outside

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• Ready for review

[daniel-wer]

Nice! Tested, works very well 👍