CI: Add golangci-lint, enforce Go best practices

[cpu]

This branch adds golangci-lint to the CI build/makefile. (Linters linting linters! 😵)

The configuration I've added (see .golangci-lint.yml) enables the following lints:

bodyclose: checks whether HTTP response body is closed successfully [fast: true, auto-fix: false]
deadcode: Finds unused code [fast: true, auto-fix: false]
dogsled: Checks assignments with too many blank identifiers (e.g. x, _, _, _, := f()) [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]
gocyclo: Computes and checks the cyclomatic complexity of functions [fast: true, auto-fix: false]
gofmt: Gofmt checks whether code was gofmt-ed. By default this tool runs with -s option to check for code simplification [fast: true, auto-fix: true]
goimports: Goimports does everything that gofmt does. Additionally it checks unused imports [fast: true, auto-fix: true]
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]
interfacer: Linter that suggests narrower interface types [fast: true, auto-fix: false]
misspell: Finds commonly misspelled English words in comments [fast: true, auto-fix: true]
nakedret: Finds naked returns in functions greater than a specified function length [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]
unconvert: Remove unnecessary type conversions [fast: true, auto-fix: false]
unparam: Reports unused function parameters [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]

I disabled a handful of others. Mostly because they are too opinionated (gocritic, whitespace, wsl, etc), not applicable (depguard, godox, gosec), or produced too many findings to address in this PR (golint, stylecheck, goconst). One day I'd like to come back around to enabling that last category when I can find the time to implement all of the renames/fixes required.

I broke up each commit in this branch based on the lint error that was being fixed. Mostly these are small diffs and a few caught real bugs (e.g. unit tests that were never run, dead code, missed err checking).

The home-spun make format-check target can be replaced entirely since it's now subsumed by the gofmt linter run by golangci-lint.