feat(sign): optimize the default value of validate sign thread number

[lurais]

What does this PR do?
Use all available cpu cores to validate transaction signatures.
Why are these changes required?

This PR has been tested by:

• Unit Tests
• Manual Testing

Follow up

Extra details

[codecov-commenter]

Codecov Report

Merging #5396 (a1b41c2) into develop (9a65bb6) will decrease coverage by 0.01%.
Report is 1 commits behind head on develop.
The diff coverage is 100.00%.

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@@              Coverage Diff              @@
##             develop    #5396      +/-   ##
=============================================
- Coverage      60.92%   60.91%   -0.01%     
  Complexity      9231     9231              
=============================================
  Files            839      839              
  Lines          50029    50029              
  Branches        5574     5574              
=============================================
- Hits           30479    30476       -3     
- Misses         17165    17167       +2     
- Partials        2385     2386       +1     

Files Changed	Coverage Δ
.../src/main/java/org/tron/core/config/args/Args.java	60.17% <100.00%> (ø)

... and 5 files with indirect coverage changes

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[tomatoishealthy]

Better remember to link the issue with this PR.

Correct to the right link if I'm wrong

[lurais]

that's right. @guoquanwu