Add gci to format imports

[sleepymole]

Currently, there is no convention on how to group the imports. Different developers may format imports to different groups. This usually results in unnecessary code changes when submitting a PR. Although we use goimports to format the imports, goimports doesn't work correctly in many scenarios, see discussion in golang/go#20818.

gci is a tool that control golang package import order and make it always deterministic. It can always sort imports into exactly two groups or three groups. And gci is also a built-in linter in golangci-lint. So I think we can add gci to our lint scripts. 2 imports groups or 3 import groups is still not yet determined.

There are some known compatibility issues for gci with other linters, see golangci/golangci-lint#1490. But most issues are very rare to encounter.