Add golangci-lint CI action, fix gosimple, govet + unused lint errors

[timvaillancourt]

Description

This PR adds a golangci-lint config and GitHub Action in order to lint the gh-ost code

Currently nothing is validating the cleanliness/best-practices of our Golang code

Initially only these linters are enabled:

1. gosimple

   Linter for Go source code that specializes in simplifying a code

2. govet

   Vet examines Go source code and reports suspicious constructs, such as Printf calls whose arguments do not align with the format string

3. unused

   Checks Go code for unused constants, variables, functions and types

And I plan to create follow-up PRs to enable the remaining linters

Introducing these 3 x linters triggered these complaints that are resolved by this PR:

Linter errors raised/resolved:

1. func REDACTED is unused (unused)
2. S1025: should use String() instead of fmt.Sprintf (gosimple)
3. S1039: unnecessary use of fmt.Sprintf (gosimple)
4. S1019: should use make([]string, len(uniqueKeyColumnNames)) instead (gosimple)
5. S1023: redundant return statement (gosimple)
6. S1007: should use raw string (...) with regexp.MustCompile to avoid having to escape twice (gosimple)
   • Suppressed error in go/mysql/instance_key.go with //nolint:gosimple because it's not a big deal
7. S1011: should replace loop with env = append(env, extraVariables...) (gosimple)
8. S1038: should use fmt.Fprintf instead of fmt.Fprintln(fmt.Sprintf(...)) (but don't forget the newline) (gosimple)
9. composites: REDACTED composite literal uses unkeyed fields (govet)
10. unreachable: unreachable code (govet)
    • Removed unused executeAndThrottleOnError func from go/logic/migrator.go

• contributed code is using same conventions as original code
• script/cibuild returns with no formatting errors, build errors or unit test errors.

[rashiq]

❤️ 🚀

[timvaillancourt]

Merging as this doesn't change logic and unit tests are passing 👍

EDIT: never mind, I don't have access to force-merge a PR with incomplete required jobs

[timvaillancourt]

@rashiq / @dm-2: there are still 2 x CI jobs that are required although we removed them in #1097 and #1101:

1. build (mysql-5.5.62)
2. build (mysql-5.6.43)

Could you help remove those 2 x stale jobs as required in the "branch protection rules" (under "Settings" for this repo) for master? 🙇

[dm-2]

@rashiq / @dm-2: there are still 2 x CI jobs that are required although we removed them in #1097 and #1101:

1. build (mysql-5.5.62)
2. build (mysql-5.6.43)

Could you help remove those 2 x stale jobs as required in the "branch protection rules" (under "Settings" for this repo) for master? 🙇

Done, mystery solved about where those jobs were configured - thanks! I've run the CI build now so you should be all good to merge once that's done 👍

[timvaillancourt]

Done, mystery solved about where those jobs were configured - thanks! I've run the CI build now so you should be all good to merge once that's done 👍

@dm-2 👋 and thanks!