[ISSUE #8336] Adjust the default value of ackMessageThreadPoolNums to 16 to prevent performance bottlenecks during high traffic

[RongtongJin]

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Which Issue(s) This PR Fixes

Fixes #8336

Brief Description

Adjust the default value of ackMessageThreadPoolNums to 16 to prevent performance bottlenecks during high traffic

How Did You Test This Change?

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 43.15%. Comparing base (b31e0ce) to head (fc947ff).

Files	Patch %	Lines
...n/java/org/apache/rocketmq/store/ConsumeQueue.java	0.00%	1 Missing :warning:

Additional details and impacted files

@@              Coverage Diff              @@
##             develop    #8337      +/-   ##
=============================================
- Coverage      43.20%   43.15%   -0.05%     
+ Complexity     10524    10511      -13     
=============================================
  Files           1274     1274              
  Lines          88940    88940              
  Branches       11434    11434              
=============================================
- Hits           38428    38384      -44     
- Misses         45808    45840      +32     
- Partials        4704     4716      +12     

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