Zarr backend follow-up: tech

[fm3]

Follow-up for reading zarr-datasets in wk backend:

• [ ] cache clear for file system handles (when password for an account changes / file system breaks somehow)
• [ ] double-check that fill value is used if chunk does not exist
• [ ] support string-encoded fill values
• [x] perf: async cache, for file systems, handles – use akka-caching (caution: Futures all around, ~needs actorSystem~)
• [x] do not depend on jzarr, instead on its dependencies itself (only those we use) https://github.com/scalableminds/webknossos/pull/6152
• [ ] better error messages when bbox is not right (e.g. assertions in computeChunkIndices)
• [ ] where to store file systems / file system provider ref
• [x] perf optimization: skip allocating buffer if single-bucket request
• [ ] perf optimization: if single-bucket request, skip typing of buffers

[fm3]

I’d say with #6152 and #6165 the two most important parts of this issue are taken care of. Let’s schedule it again when we feel that the other checkboxes are relevant enough :)

[fm3]

I split the remaining parts of this issue into #6989 and #6990