Enable perfsprint linter and fix up code issues

[alexandear]

This PR enables perfsprint linter and fixes its issues.

Changes that improve both readability and performance are:

• replace fmt.Sprintf("%d", x), fmt.Sprint(x) with strconv.Itoa(x) for int values;
• replace fmt.Sprintf("%t", b) with strconv.FormatBool(b) for bool values;
• replace fmt.Sprintf("%d", x) with strconv.FormatInt for int64 values;
• replace fmt.Sprintf("%d", x) with strconv.FormatUint for uint64 values;
• remove redundant fmt.Sprint statements.

[beorn7]

Looks good, just wondering about the text entry in .golangci.yml.

[machine424]

Thanks @alexandear for this and @beorn7 for the review.