Minor performance enhancements

[gammazero]

• Do not reset idle timer after every task
• Idle timer timeout should be const, not member var
• All callers of Stop and StopWait will wait for WorkerPool to stop
• Faster test execution
• Function decomposition for readability

[codecov-commenter]

Codecov Report

Merging #31 into master will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #31   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines          123       132    +9     
=========================================
+ Hits           123       132    +9     

Impacted Files	Coverage Δ
workerpool.go	100.00% <100.00%> (ø)

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