get rid of sync.Map for better go version compatibility

[zhijinli]

trying to resolve #7 to support Go 1.8

[codecov]

Codecov Report

Merging #13 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #13      +/-   ##
==========================================
+ Coverage   65.97%   66.05%   +0.07%     
==========================================
  Files          20       20              
  Lines        1743     1747       +4     
==========================================
+ Hits         1150     1154       +4     
  Misses        511      511              
  Partials       82       82

Impacted Files	Coverage Δ
participant.go	75.48% <100%> (+0.21%)	⬆️

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[syyang]

nit: let's avoid inline comment

[syyang]

We should stick to 1.9 unless there's a huge demand for supporting 1.8.

[zhijinli]

I'm not sure about the tradeoff between supporting more versions and losing the 1.9 specific features, but I agree we can punt on this PR and evaluate the tradeoff when demand for 1.8 (or below) really comes.

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