fix: always copy once after top get

[AsterDY]

Background

sonic.Get() does copy the entired JSON and refer it to returned partial json. This behavior is neither CPU-friendly nor memory-safe: Copy entire JSON is time consuming, and returned JSON consumes more memory especially if it is cached or hold for a long term. Thus, We decide to not copy input JSON bytes, but copy returned partial JSON instead.

What's the effect

After this change, most normal services may gain a little CPU improve on sonic.Get(), and cache-like services will be less possible to be OOM.
Only problem is for services who exists data-race on input JSON bytes, the panic may be more frequent. However, we believe this is a rare case in practice, and we DONOT expect any data race on input JSON.

[codecov-commenter]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (63f4c07) 78.57% compared to head (ae7e496) 74.26%.

Files	Patch %	Lines
ast/search.go	70.83%	4 Missing and 3 partials ⚠️
api.go	60.00%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #577      +/-   ##
==========================================
- Coverage   78.57%   74.26%   -4.32%     
==========================================
  Files          69       63       -6     
  Lines       10823     8435    -2388     
==========================================
- Hits         8504     6264    -2240     
+ Misses       1942     1830     -112     
+ Partials      377      341      -36     

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