[MINOR] Fix inefficient map iteration

[kaijchen]

What changes were proposed in this pull request?

Iterate Map by Map.entrySet() instead of Map.keySet() + Map.get(key).

Why are the changes needed?

It's more efficient.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

CI.

[codecov-commenter]

Codecov Report

Merging #245 (5e0dd40) into master (75921f1) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #245      +/-   ##
============================================
+ Coverage     59.10%   59.18%   +0.08%     
- Complexity     1330     1333       +3     
============================================
  Files           160      160              
  Lines          8740     8740              
  Branches        821      821              
============================================
+ Hits           5166     5173       +7     
+ Misses         3307     3302       -5     
+ Partials        267      265       -2     

Impacted Files	Coverage Δ
...nator/LowestIOSampleCostSelectStorageStrategy.java	74.07% <100.00%> (+0.92%)	⬆️
...pache/hadoop/mapreduce/task/reduce/RssFetcher.java	90.62% <0.00%> (+1.56%)	⬆️
...org/apache/uniffle/server/ShuffleFlushManager.java	78.33% <0.00%> (+1.66%)	⬆️
...storage/handler/impl/DataSkippableReadHandler.java	84.37% <0.00%> (+3.12%)	⬆️

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[zuston]

Thanks for your contribution! @kaijchen

[kaijchen]

Thanks @zuston for the review.