Do not use closures to prevent some data races

[leonklingele]

In order to permanently avoid the problem of #2396, we should start and switch to never return anonymous functions (closures) but return non-anonymous functions instead so no data is shared — never. For middlewares this might look as follows:

// New creates a new middleware handler
func New(config ...Config) fiber.Handler {
	// Setup, etc.

	// Return new handler
	return newHandler(list, shared, data, here, config, etc)
}

func newHandler(list, shared, data, here, config, etc) func(c *fiber.Ctx) {
    // Important: Never do anything in here before returning the closure.
    return func(c *fiber.Ctx) {
        // Code of handler as before, but without access to data of setup logic
    }
}

Originally posted by @leonklingele in #2396 (comment)

[ReneWerner87]

we should definitely check the benchmarks afterwards to make sure that the allocations have remained identical.

Since the framework is trimmed for speed, this has priority over possible error prevention in this case.
but hope this should not conflict at all and of course works without changing the memory accesses

[leonklingele]

we should definitely check the benchmarks afterwards to make sure that the allocations have remained identical.

Go should inline those calls. We can verify using https://github.com/nikolaydubina/go-recipes#-show-compiler-optimization-decisions-on-heap-and-inlining