Java: Adjust default thread num of threadpool

[aaron-ai]

No description provided.

[codecov-commenter]

Codecov Report

Merging #247 (da795ee) into master (170b2f0) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #247   +/-   ##
=========================================
  Coverage     35.84%   35.85%           
  Complexity      662      662           
=========================================
  Files           220      220           
  Lines         11229    11229           
  Branches        275      275           
=========================================
+ Hits           4025     4026    +1     
+ Misses         6957     6955    -2     
- Partials        247      248    +1     

Flag	Coverage Δ
java	62.23% <100.00%> (+0.10%)	⬆️

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Impacted Files	Coverage Δ
...g/apache/rocketmq/client/java/impl/ClientImpl.java	55.30% <100.00%> (+1.28%)	⬆️
...e/rocketmq/client/java/impl/ClientManagerImpl.java	82.26% <100.00%> (ø)
cpp/source/admin/include/ServerCall.h	85.71% <0.00%> (-14.29%)	⬇️

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