fix concurrent modification on gc

[skhugh]

What this PR does / why we need it?

fix concurrent modification on gc.
I used iterator for textWithGarbageSetByCreatedAt, but snapshot for removedElementSetByCreatedAt because gc flow is quite complex here.

Any background context you want to provide?

What are the relevant tickets?

Fixes #

Checklist

• Added relevant tests or not required
• Didn't break anything

[codecov]

Codecov Report

Merging #42 (753dc4d) into main (e18bfe2) will increase coverage by 0.44%.
The diff coverage is 55.55%.

@@             Coverage Diff              @@
##               main      #42      +/-   ##
============================================
+ Coverage     75.65%   76.09%   +0.44%     
- Complexity      369      371       +2     
============================================
  Files            43       43              
  Lines          1614     1615       +1     
  Branches        230      231       +1     
============================================
+ Hits           1221     1229       +8     
+ Misses          259      249      -10     
- Partials        134      137       +3     

Impacted Files	Coverage Δ
...c/main/kotlin/dev/yorkie/document/crdt/CrdtRoot.kt	58.53% <55.55%> (+9.15%)	⬆️

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[hackerwins]

Thanks for your contribution. 👍