refactor: use *http.ServeMux

[dwisiswant0]

Make *http.Server.Shutdown works with *http.ServeMux.

So it can defer the *Statistics.Stop (projectdiscovery/httpx#1856).

[Mzack9999]

lgtm!

Unrelated to this PR, but what do you think about also using something like this within the start:

errChan := make(chan error, 1)
var done atomic.Bool

go func() {
	if err := s.httpServer.ListenAndServe(); err != nil && err != http.ErrServerClosed && !done.Load() {
		errChan <- err
	}
}()

// catch initial fatal errors
select {
	case err := <-errChan:
		return err
	case <-time.After(250 * time.Millisecond):
		done.Store(true)
		close(errChan)
}

to catch s.httpServer.ListenAndServe() errors in Statistics.Start(...)

[dwisiswant0]

@Mzack9999 - I was thinking about immediately returning ListenAndServe and invoking Statistics.Start using goroutine. It seems like this approach needs to be tested out.