Run more linters with golangci-lint

Currently we run only `golint` and `gofmt`. We should at least enable linters which are enabled by default in `golangci-lint`:
```
$ golangci-lint help linters
Enabled by default linters:
deadcode: Finds unused code [fast: true, auto-fix: false]
errcheck: Errcheck is a program for checking for unchecked errors in go programs. These unchecked errors can be critical bugs in some cases [fast: true, auto-fix: false]
gosimple (megacheck): Linter for Go source code that specializes in simplifying a code [fast: true, auto-fix: false]
govet (vet, vetshadow): Vet examines Go source code and reports suspicious constructs, such as Printf calls whose arguments do not align with the format string [fast: true, auto-fix: false]
ineffassign: Detects when assignments to existing variables are not used [fast: true, auto-fix: false]
staticcheck (megacheck): Staticcheck is a go vet on steroids, applying a ton of static analysis checks [fast: true, auto-fix: false]
structcheck: Finds unused struct fields [fast: true, auto-fix: false]
typecheck: Like the front-end of a Go compiler, parses and type-checks Go code [fast: true, auto-fix: false]
unused (megacheck): Checks Go code for unused constants, variables, functions and types [fast: false, auto-fix: false]
varcheck: Finds unused global variables and constants [fast: true, auto-fix: false]
```

Enabling some of them (I've checked `errcheck` and `deadcode`) would require fixing outstanding issues.

Work here should be done incrementally, one-by-one.

/good-first-issue
/help-wanted