Fix prefetchNextBucketEntries so it fetches the correct next 2 buckets.

[Nicky-2000]

This PR fixes a bug in prefetchNextBucketEntries which is used in the hashtable iterator. The current version of prefetchNextBucketEntries does not correctly prefetch the next two buckets that will be iterated over. This is due next_index being incremented twice (once in prefetchNextBucketEntries and again in getNextBucket).

Fix Explanation:

1. Add check in prefetchNextBucketEntries to ensure we pass the correct 'next_index' to getNextBucket.
2. The extra increment was removed from getNextBucket.

Performance

The fix was tested in a similar manner as #1501 where these changes were introduced.

All the data below was taken on a
The data below was collected by executing the KEYS * command and Save command on a 64-logical core (32 physical cores, 2 threads per core) AMD EPYC 7B13 processor. The dataset had 50 million keys with a value size of 100 bytes each.

KEYS * Command

Fix Applied	Mean Time (seconds) Over 8 trials	Std Dev (s) Over 8 Trials
No	6.374759	0.10
Yes	6.184731	0.06

Improvement:

• Modest 2.98% reduction in command duration,
• 40% improvement in standard deviation.
• This shows that the fix slightly speeds up the command and makes its performance more consistent.

Save command improvement

Setup:

• 64-logical core (32 physical cores, 2 threads per core) AMD EPYC 7B13
• 50 mil keys in size of 100 bytes each.
• Running valkey server over standard file system with 402 M/s throughput.
• crc checksum ON and comperssion ON .

Results

Fix Applied	Mean Time (s) Over 8 trials	Std Dev (s) Over 8 Trials
No	49.87	0.38
Yes	49.85	0.15

Improvement:

• No noticeable improvement or regression in average Save performance
• Performance is more consistent with the fix (standard deviation 62% smaller over the 8 test trials).

[zuiderkwast]

@NadavGigi PTAL

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.49%. Comparing base (a481fe2) to head (57d1964).
⚠️ Report is 18 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #2394      +/-   ##
============================================
+ Coverage     71.44%   71.49%   +0.05%     
============================================
  Files           123      123              
  Lines         67177    67179       +2     
============================================
+ Hits          47992    48032      +40     
+ Misses        19185    19147      -38     

Files with missing lines	Coverage Δ
src/hashtable.c	82.84% <100.00%> (+0.41%)	⬆️

... and 14 files with indirect coverage changes

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