重构context对象

[guonaihong]

#132 重构context对象，让context对象包含编码器和解码器。为的是和filter层打通。

[codecov-io]

Codecov Report

Merging #161 into master will increase coverage by 0.04%.
The diff coverage is 81.81%.

@@            Coverage Diff             @@
##           master     #161      +/-   ##
==========================================
+ Coverage   89.42%   89.46%   +0.04%     
==========================================
  Files          41       40       -1     
  Lines        1673     1671       -2     
==========================================
- Hits         1496     1495       -1     
  Misses        128      128              
+ Partials       49       48       -1

Impacted Files	Coverage Δ
dataflow/req.go	82.35% <81.81%> (-0.37%)	⬇️
bench/task.go	86.58% <0%> (-1.22%)	⬇️
dataflow/debug.go	94.11% <0%> (+5.88%)	⬆️

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