update ESLint and plugins/configs

[tinovyatkin]

1. Updates ESLint to the version 6.5.x
2. Updates eslint-config-standard to the version 14.x from 7.x. It brings two more required peerDependencies, eslint-plugin-import and eslint-plugin-node, and the last one if very helpful with Koa tricky engines
3. Add explicit extension to the eslint config for tests, lowering newly discovered errors to warning for now, and fixing env from jest to mocha
4. Fixing lint errors in benchmark.

[codecov]

Codecov Report

Merging #1407 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1407   +/-   ##
=======================================
  Coverage   99.36%   99.36%           
=======================================
  Files           4        4           
  Lines         474      474           
  Branches      125      125           
=======================================
  Hits          471      471           
  Partials        3        3

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