builder: delay free data structure to reduce image build time

[jiangliu]

According to perf flame graph, it takes a long time to free objects used by image builder. In most common cases, the builder will only run once and exit, so it's unnecessary to free those used objects.

Types of changes

What types of changes does your PullRequest introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation Update (if none of the other choices apply)

Checklist

Go over all the following points, and put an x in all the boxes that apply.

• I have updated the documentation accordingly.
• I have added tests to cover my changes.

[anolis-bot]

@jiangliu , a new test job has been submitted. Please wait in patience. The test job url: https://tone.openanolis.cn/ws/nrh4nnio/test_result/74684

[codecov]

Codecov Report

Merging #1298 (c8d460a) into master (809f8d9) will increase coverage by 0.00%.
The diff coverage is 50.00%.

@@           Coverage Diff           @@
##           master    #1298   +/-   ##
=======================================
  Coverage   46.03%   46.03%           
=======================================
  Files         124      124           
  Lines       36934    36953   +19     
  Branches    36934    36953   +19     
=======================================
+ Hits        17004    17013    +9     
- Misses      19041    19052   +11     
+ Partials      889      888    -1     

Impacted Files	Coverage Δ
rafs/src/builder/core/tree.rs	72.41% <0.00%> (-0.42%)	⬇️
rafs/src/builder/directory.rs	0.00% <0.00%> (ø)
rafs/src/builder/stargz.rs	0.00% <0.00%> (ø)
src/bin/nydus-image/main.rs	40.80% <0.00%> (-0.12%)	⬇️
rafs/src/builder/tarball.rs	63.06% <100.00%> (+0.16%)	⬆️
utils/src/lib.rs	84.33% <100.00%> (+1.44%)	⬆️

... and 2 files with indirect coverage changes

[anolis-bot]

@jiangliu , The CI test is completed, please check result:

Test Case		Test Result
build rust golang image		✅ SUCCESS
compile nydusd		✅ SUCCESS
compile ctr remote		✅ SUCCESS
compile nydus snapshotter		✅ SUCCESS
run container with rafs		✅ SUCCESS
run container with zran		✅ SUCCESS
run container with rafs and compile linux		✅ SUCCESS

Congratulations, your test job passed!

[anolis-bot]

@adamqqqplay , the title has been updated, so a new test job has been submitted. Please wait in patience. The test job url: https://tone.openanolis.cn/ws/nrh4nnio/test_result/75278

[anolis-bot]

@adamqqqplay , The CI test is completed, please check result:

Test Case		Test Result
nydus_ci		❌ FAIL

Sorry, your test job failed. Please get the details in the link.