global concurrent limiter

全局的并发请求限制，可以用于控制应用的并发请求量。

Example

package main
import (
    "bytes"
    "sync"
    "time"
    "github.com/vicanso/elton"
    "github.com/vicanso/elton/middleware"
)
func main() {
    e := elton.New()
    e.Use(middleware.NewGlobalConcurrentLimiter(middleware.GlobalConcurrentLimiterConfig{
        Max: 1000,
    }))
    e.POST("/login", func(c *elton.Context) (err error) {
        time.Sleep(3 * time.Second)
        c.BodyBuffer = bytes.NewBufferString("hello world")
        return
    })
    err := e.ListenAndServe(":3000")
    if err != nil {
        panic(err)
    }
}