fix: Use localStorage for memqueue tests instead of Redis for #162

[shaunco]

Structured similarly to how #106 moved storage to storage.go, but I made a few changes:

• Added NewLocalStorage() function to allow taskq users to do opt.Storage = taskq.NewLocalStorage(), as I agree with your comment in fixed panics on msg.Name != "" & redis is disabled #106 that introducing a new default will be unexpected by most and I'd also rather see an error when storage is not configured
• NewLocalStorage() and newRedisStorage() return Storage interfaces instead of their internal types
• Exist methods take pointers for their receiver argument, since LocalStorage would otherwise pass its sync.Mutex by value

[shaunco]

Let me know if you want me to fix the commitlint complaint (subject must not be sentence-case, start-case, pascal-case, upper-case [subject-case]), even though it is complaining about localStorage and Redis, which should have those specific cases 😩

[vmihailenco]

Thanks 👍