## Description of changes

_Summarize the changes made by this PR._

- Improvements & Bug fixes
- We recently changed get_prefix() to use buffered_unordered().
Previously it was join_all(). The assumption was that upstream callsites
don't expect results in sorted order of keys
- However, sparse writer expects its keys in order for e.g. when
computing the block maxes of a dimension; it does a
get_prefix(encoded_dim) and expects the block maxes to be in sorted
order of the block end offset id.
    - Fixed by changing to buffered()
- New functionality
    - ...

## Test plan

_How are these changes tested?_

- [x] Tests pass locally with `pytest` for python, `yarn test` for js,
`cargo test` for rust

## Migration plan

_Are there any migrations, or any forwards/backwards compatibility
changes needed in order to make sure this change deploys reliably?_

## Observability plan

_What is the plan to instrument and monitor this change?_

## Documentation Changes

_Are all docstrings for user-facing APIs updated if required? Do we need
to make documentation changes in the_ [_docs
section](https://github.com/chroma-core/chroma/tree/main/docs/docs.trychroma.com)?_

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Co-authored-by: Sanket Kedia <sanketkedia@Sankets-MBP.hsd1.ca.comcast.net>