Add Apache 2 license header to every Go file

[ytsarev]

• Add Apache 2 headers
• Add incredible piece of automation for header injection

[kuritka]

I dind't study deepply, but there are tools doing that: https://github.com/cloudposse/copyright-header. Maybe we can manipulate with headers before GOLINT

[ytsarev]

@kuritka I definitely don't want to employ super sophisticated machinery with lots of rubygem dependencies here.

Also tried https://github.com/google/addlicense . It does pretty much the same stuff as the tiny shell script provided.

So I suggest we stick with the simplest version of it which does not bring in any new dependencies