feat: Implement memtable range

[evenyag]

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What's changed and what's your intention?

This PR implements MemtableRange to represent a partition (range) of a memtable. This allows iterating the memtable concurrently. The memtable range keeps an IterBuilder to build an iterator to read that range.

The Memtable trait has a new method ranges() to return the list of ranges to read. Both time_series and partition_tree memtables return just 1 range.

The ScanPart now keeps a list of MemtableRange instead of Memtable.

It also refactors the FileRange and extracts a struct RangeBase so we can reuse it in other places. The RangeBase contains fields to prune batches from a file.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.

[v0y4g3r]

Is it possible to completely deprecate Memtable::iter with Memtable::range? I think range is more generic and compatible with current RegionScanner design. In the long term, memtable should be considered as an in-memory file part just like those in disks.

[codecov]

Codecov Report

Attention: Patch coverage is 86.66667% with 22 lines in your changes missing coverage. Please review.

Project coverage is 84.59%. Comparing base (0fc18b6) to head (3e00faf).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4162      +/-   ##
==========================================
- Coverage   84.92%   84.59%   -0.34%     
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  Files        1023     1024       +1     
  Lines      179298   179502     +204     
==========================================
- Hits       152272   151846     -426     
- Misses      27026    27656     +630     

[v0y4g3r]

LGTM

[evenyag]

Is it possible to completely deprecate Memtable::iter with Memtable::range?

It should be possible. Do we need to do it in this PR? Maybe we can deprecate iter() after ranges() is stable.

[killme2008]

LGTM