router concurrent limiter

路由限制中间件，可以指定路由的并发访问数量，建议使用NewLocalLimiter每个实例的限制分开，主要是用于避免某个接口并发过高导致系统不稳定。

Example

package main
import (
    "bytes"
    "time"
    "github.com/vicanso/elton"
    "github.com/vicanso/elton/middleware"
)
func main() {
    e := elton.New()
    e.Use(middleware.NewRCL(middleware.RCLConfig{
        Limiter: middleware.NewLocalLimiter(map[string]uint32{
            "GET /users/me": 2,
        }),
    }))
    e.GET("/users/me", func(c *elton.Context) (err error) {
        time.Sleep(time.Second)
        c.BodyBuffer = bytes.NewBufferString(`{
            "account": "tree",
            "name": "tree.xie"
        }`)
        return nil
    })
    err := e.ListenAndServe(":3000")
    if err != nil {
        panic(err)
    }
}