perf(etcd): refactor some code to improve performance

[xuruidong]

Description

Fixes # (issue)

Before:
The original code performed a cleanup of self.values and rebuilt self.values_hash by:

• Made a full copy of self.values
• Cleared the original and reinserted non-nil values
• Rebuilt values_hash in a separate step

Problems:

• Wasted memory (copied entire table)
• Slow (processed data twice)

Now:

1. In-place cleanup:
   • Removes nil values while keeping the same table
   • Updates values_hash at the same time
2. Trims empty space:
   • Deletes unused slots at the end

Why better?

• Faster: Does everything in one pass
• Uses less memory: No extra table copies
• Works better for big datasets

Before:
The original code performed a cleanup of self.values and rebuilt self.values_hash by:

• Made a full copy of self.values
• Cleared the original and reinserted non-nil values
• Rebuilt values_hash in a separate step

Problems:

• Wasted memory (copied entire table)
• Slow (processed data twice)

Now:

1. In-place cleanup:
   • Removes nil values while keeping the same table
   • Updates values_hash at the same time
2. Trims empty space:
   • Deletes unused slots at the end

Why better?

• Faster: Does everything in one pass
• Uses less memory: No extra table copies
• Works better for big datasets

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[juzhiyuan]

@xuruidong Just approved to run the CI

[xuruidong]

@xuruidong Just approved to run the CI

please run the CI again @juzhiyuan

[juzhiyuan]

@xuruidong Just approved to run the CI

please run the CI again @juzhiyuan

Hi @xuruidong, I couldn't find the Re-run button. Can you confirm the failed test cases?

[xuruidong]

I don't think the CI failure is related to the code changes.

[Baoyuantop]

Hi @xuruidong, can you add some descriptive information for PR?

[Baoyuantop]

Hi @xuruidong, please add a PR description so that others can review it better ~

[xuruidong]

Hi @xuruidong, please add a PR description so that others can review it better ~

Hi @Baoyuantop , the PR description has been added, please help review it. Thanks.