fix: memory leak when handling large resp body

[shreemaan-abhishek]

Description

Fixes # (issue)

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)

[membphis]

@shreemaan-abhishek

Is this task still ongoing? If not, we can close it ^_^

[shreemaan-abhishek]

@membphis, yes I still want to work on it, I am unable to repro the bug right now 😂.

In theory, the memory leak still exists, the method of body chunk collection is wrong, and can lead to memory leak.

check: openresty/lua-nginx-module#2346

[membphis]

I can sure this is not a memory leak case

the memory was cache to the dlmalloc, It does not release the memory immediately.