make Context.Value method concurrent safe

[fengyoulin]

Make Context.Value, Context.Get, and Context.Set methods concurrent safe, to be consistent with Go's context.Context.

[codecov]

Codecov Report

Merging #2183 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #2183      +/-   ##
=========================================
+ Coverage   98.49%   98.5%   +<.01%     
=========================================
  Files          40      40              
  Lines        2257    2272      +15     
=========================================
+ Hits         2223    2238      +15     
  Misses         18      18              
  Partials       16      16

Impacted Files	Coverage Δ
logger.go	100% <100%> (ø)	⬆️
context.go	98.68% <100%> (+0.02%)	⬆️

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[fengyoulin]

#700

[bestgopher]

#700
when the context has to be passed to a goroutine,we should use context.Copy get a new context。

[appleboy]

duplicated of #1391