concurrent limiter

并发请求限制，可以通过指定请求的参数，如IP、query的字段或者body等获取，限制同时并发性的提交请求，主要用于避免相同的请求多次提交。指定的Key分为以下几种：

• :ip 客户的RealIP
• h:key 从HTTP请求头中获取key的值
• q:key 从HTTP的query中获取key的值
• p:key 从路由的params中获取key的值
• 其它的则从HTTP的Post data中获取key的值（只支持json)

Example

package main
import (
    "bytes"
    "sync"
    "time"
    "github.com/vicanso/elton"
    "github.com/vicanso/elton/middleware"
)
func main() {
    e := elton.New()
    m := new(sync.Map)
    limit := middleware.NewConcurrentLimiter(middleware.ConcurrentLimiterConfig{
        Keys: []string{
            ":ip",
            "h:X-Token",
            "q:type",
            "p:id",
            "account",
        },
        Lock: func(key string, c *elton.Context) (success bool, unlock func(), err error) {
            _, loaded := m.LoadOrStore(key, true)
            // the key not exists
            if !loaded {
                success = true
                unlock = func() {
                    m.Delete(key)
                }
            }
            return
        },
    })
    e.POST("/login", limit, 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)
    }
}