Improve handling of tasks submitted to a stopped pool

[alitto]

Changes included

• Improve handling of tasks submitted to a stopped pool
  • Sending a task using Submit() will panic with ErrSubmitOnStoppedPool if the pool has been stopped.
  • Sending a task using TrySubmit() will return false if the pool has been stopped.
• Update dependencies in benchmark package

[codecov-commenter]

Codecov Report

Merging #22 (f833392) into master (a646d99) will increase coverage by 0.04%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #22      +/-   ##
==========================================
+ Coverage   98.52%   98.57%   +0.04%     
==========================================
  Files           2        2              
  Lines         203      210       +7     
==========================================
+ Hits          200      207       +7     
  Misses          3        3              

Impacted Files	Coverage Δ
pond.go	98.47% <100.00%> (+0.05%)	:arrow_up:

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