[#889] improvement: Modify default value of single buffer flush.

[leixm]

What changes were proposed in this pull request?

Modify default value of single buffer flush.

Why are the changes needed?

For release 0.8.0.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

exiting UTs.

[codecov-commenter]

Codecov Report

Merging #1039 (3910376) into master (96eab1c) will increase coverage by 1.44%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #1039      +/-   ##
============================================
+ Coverage     54.32%   55.76%   +1.44%     
+ Complexity     2550     2413     -137     
============================================
  Files           382      349      -33     
  Lines         21737    18003    -3734     
  Branches       1803     1684     -119     
============================================
- Hits          11808    10039    -1769     
+ Misses         9228     7365    -1863     
+ Partials        701      599     -102     

Files Changed	Coverage Δ
...a/org/apache/uniffle/server/ShuffleServerConf.java	99.43% <100.00%> (ø)
...he/uniffle/server/buffer/ShuffleBufferManager.java	84.27% <100.00%> (+0.09%)	⬆️

... and 36 files with indirect coverage changes

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[zuston]

LGTM