enable codecov partial coverage and use bash uploader

[tinovyatkin]

Enables partial line coverage for Codecov and uses faster Bash uploader targeting just one particular file

This will lower reported coverage because CI tests currently runs only on Node >=8 and Koa has some legacy code for earlier versions of Node

[codecov]

Codecov Report

Merging #1396 into master will decrease coverage by 0.62%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1396      +/-   ##
==========================================
- Coverage     100%   99.37%   -0.63%     
==========================================
  Files           4        4              
  Lines         382      477      +95     
  Branches        0      127     +127     
==========================================
+ Hits          382      474      +92     
- Partials        0        3       +3

Impacted Files	Coverage Δ
lib/application.js	98.19% <0%> (-1.81%)	⬇️
lib/response.js	99.37% <0%> (-0.63%)	⬇️
lib/request.js	100% <0%> (ø)	⬆️
lib/context.js	100% <0%> (ø)	⬆️

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