Fix cache mutex lock

[apoq]

Hello. There's a problem with mutex lock in fiber cache middleware. See the example below:

package main

import (
	"github.com/gofiber/fiber/v2"
	"github.com/gofiber/fiber/v2/middleware/cache"
	_ "net/http/pprof"
	"time"
)

func main() {
	app := fiber.New()

	app.Use(cache.New(cache.Config{
		Next: func(c *fiber.Ctx) bool {
			if c.Route().Path == "/health" {
				return false
			}

			return true
		},
	}))

	app.Get("/health", func(c *fiber.Ctx) error {
		return c.SendString("ok")
	})

	app.Get("/test", func(c *fiber.Ctx) error {
		time.Sleep(5 * time.Second)
		return c.SendString("sleep-test")
	})

	if err := app.Listen(":9001"); err != nil {
		panic(err)
	}
}

Try accessing /test and /health endpoints. Health endpoint will respond only when Test endpoint has finished. Thus, the slowest endpoint becomes the bottleneck for all GET requests (as other methods are being skipped in cache middleware)

This PR makes sure that we don't have lock enabled when running c.Next() (down the stack), and only being used when dealing with cache entry.

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