refactor: make some variables private in struct & allow limiting cache size & CacheByRequestBody

[huantt]

Some variable should be private in struct because you may replace it by other ones.

[chenyahui]

CacheByRequestBody is interesting. And I left some code review in this pr~

[codecov-commenter]

Codecov Report

Merging #26 (887cd09) into main (b885ae3) will increase coverage by 0.42%. The diff coverage is 61.53%.

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@@            Coverage Diff             @@
##             main      #26      +/-   ##
==========================================
+ Coverage   63.33%   63.76%   +0.42%     
==========================================
  Files           5        5              
  Lines         270      287      +17     
==========================================
+ Hits          171      183      +12     
- Misses         86       90       +4     
- Partials       13       14       +1     

Flag	Coverage Δ
unittests	63.76% <61.53%> (+0.42%)	:arrow_up:

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Files Changed	Coverage Δ
persist/redis.go	0.00% <0.00%> (ø)
persist/memory.go	83.33% <57.14%> (-7.58%)	:arrow_down:
cache.go	71.95% <80.00%> (+0.81%)	:arrow_up:

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[huantt]

@chenyahui I have just updated my PR, please review it.