try to fix socket i/o error for session

[zensh]

If socket was killed, it should be removed in session so that new socket can be setup for session.

[zensh]

@niemeyer https://travis-ci.org/go-mgo/mgo/jobs/289072737 this job failed, it should be restart~

[zensh]

After this PR，we can use:

s := session.Clone()
defer s.Close()

instead of

s := session.Copy()
defer s.Close()

[zensh]

We can try the issue with code:

func main() {
	session, err := mgo.DialWithInfo(someRemoteDailInfo)
	if err != nil {
		panic(err)
	}

	for {
		fmt.Println("Ping:", session.Ping())
		time.Sleep(time.Second)
	}
}

Run it, then close the wifi, and then open the wifi, some result like this:

Ping: <nil>
Ping: <nil>
Ping: <nil>
Ping: <nil>
Ping: <nil>
Ping: <nil>
Ping: <nil>
Ping: read tcp 192.168.0.72:58883->192.168.0.21:27017: i/o timeout
Ping: no reachable servers
Ping: <nil>
Ping: <nil>
Ping: <nil>

It means that the session will reconnect automatic.

[zensh]

@niemeyer Could you review this PR? We have used the fork version in our productions