Support udp reconnection

[guidao]

For a "connected UDP socket", write will return an error when the udp port is unreachable, so it might be possible to perform a reconnection based on this error.

package main

import (
        "fmt"
        "net"
        "time"
)

func main() {
        conn, err := net.Dial("udp", "127.0.0.1:9999")
        if err != nil {
                panic(err)
        }
        for {
                n, err := conn.Write([]byte("xxx.yyy.zzz.count:1|c"))
                if err != nil {
                        fmt.Println("conn write failed:", err)
                } else {
                        fmt.Println("conn write byte:", n)
                }
                time.Sleep(time.Second)
        }
}

result:

conn write byte: 21
conn write failed: write udp 127.0.0.1:62924->127.0.0.1:9999: write: connection refused
conn write byte: 21
conn write failed: write udp 127.0.0.1:62924->127.0.0.1:9999: write: connection refused
conn write byte: 21

This feature is useful when the dns changes

[blakearnold]

Hi,
I opened #280 which should resolve this. Let me know if we can merge.