[ISSUE-173][FOLLOWUP] The size of single buffer flush should reach rss.server.flush.cold.storage.threshold.size

[leixm]

What changes were proposed in this pull request?

follow #173, when single buffer flush is triggered, the buffer size should reach rss.server.flush.cold.storage.threshold.size, we keep the same logic as MultiStorageManager#selectStorageManager

Why are the changes needed?

Make sure cold storage is valid

Does this PR introduce any user-facing change?

No

How was this patch tested?

Already added

[codecov-commenter]

Codecov Report

Merging #180 (aec72e1) into master (6cbd9b7) will increase coverage by 0.02%.
The diff coverage is 84.61%.

@@             Coverage Diff              @@
##             master     #180      +/-   ##
============================================
+ Coverage     58.38%   58.40%   +0.02%     
- Complexity     1266     1269       +3     
============================================
  Files           158      158              
  Lines          8417     8426       +9     
  Branches        780      782       +2     
============================================
+ Hits           4914     4921       +7     
- Misses         3251     3253       +2     
  Partials        252      252              

Impacted Files	Coverage Δ
...che/uniffle/server/storage/HdfsStorageManager.java	90.00% <83.33%> (-2.69%)	⬇️
...he/uniffle/server/buffer/ShuffleBufferManager.java	82.77% <100.00%> (ø)

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[jerqi]

Thanks @leixm ,Merged.