Allow concurrent calls to StopAndWait

[alitto]

Changes included

• Allow concurrent calls to StopAndWait
• Restore semantics of Stop method, which shouldn't wait for queued tasks to complete before killing worker goroutines
• Simplify logic in submit function
• Prevent hits counter in RatedResizer from going to negative values
• Add more tests

[codecov-commenter]

Codecov Report

Merging #28 (b52e832) into master (23a42e4) will increase coverage by 0.30%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master       #28      +/-   ##
===========================================
+ Coverage   99.69%   100.00%   +0.30%     
===========================================
  Files           2         2              
  Lines         328       329       +1     
===========================================
+ Hits          327       329       +2     
+ Misses          1         0       -1     

Impacted Files	Coverage Δ
pond.go	100.00% <100.00%> (+0.32%)	:arrow_up:
resizer.go	100.00% <100.00%> (ø)

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