[Util] add nWorker to verify execution result

[zhangchiqing]

Speed up the verify execution result command by adding a nWorker parameter which allows the verification to run concurrently.

a n2-highcpu-32 used to run at 1.5 block / second, but with this change a n2-highcpu-48 instance running with nWorker=30 can complete 10K blocks verification in 3 mins, which is 55 blocks / second.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 76.31579% with 18 lines in your changes missing coverage. Please review.

Project coverage is 41.00%. Comparing base (45950a7) to head (05c552f).

Files with missing lines	Patch %	Lines
cmd/util/cmd/verify_execution_result/cmd.go	0.00%	9 Missing ⚠️
engine/verification/verifier/verifiers.go	86.56%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6830      +/-   ##
==========================================
+ Coverage   40.98%   41.00%   +0.02%     
==========================================
  Files        2103     2103              
  Lines      184900   184963      +63     
==========================================
+ Hits        75783    75850      +67     
+ Misses     102750   102746       -4     
  Partials     6367     6367              

Flag	Coverage Δ
unittests	41.00% <76.31%> (+0.02%)	⬆️

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