investigate and fix golangci-lint issues (without recheck, unused, and staticcheck)

[dmitris]

golangci-lint run -D errcheck -D unused -D staticcheck ./... output: https://gist.github.com/dmitris/00c875553d64f2805c95c0c787c8ae0f

It would be good to go through them and fix those that are relevant (and possibly add //nolint:<issuetype> to those that we decide not to fix). The staticcheck issues are covered in #3519.