Enable perfsprint linter; fix up lint issues

[alexandear]

The PR enables the perfsprint linter in the golangci-lint configuration file. The fasthttp library is heavily focused on performance, making this linter a beneficial addition.

This PR fixes up the lint issues that arose from persprint. All of these were minor:

• replace fmt.Errorf("msg") with errors.New("msg));
• replace fmt.Sprintf("%d", n) with strconv.Itoa(n) in tests.

[erikdubbelboer]

Thanks!