Add ConnectionClosedCallback

[SimoneDutto]

Hello,
I am trying to implement a MaxConcurrentConnections for the ssh server.
I've notice the library already offers a ConnCallback and ConnectionFailedCallback.
However that seems not to be enough to create a guard around the number of maximum connection because we are missing the ConnectionClosedCallback.

With something like ConnectionClosedCallback I could simply implement my rate limiter by doing:

ssh.Server{
  ConnCallback: func(ctx ssh.Context, conn net.Conn) net.Conn {
	mux.Lock()
        defer mux.Unlock()
        if n > maxConnections {
		conn.Close()
		return conn
	}
	n ++
        return conn
  },
  ConnectionFailedCallback: func(conn net.Conn, err error) {
        mux.Lock()
        defer mux.Unlock()
  	n--
  },
  ConnectionClosedCallback: func(){
        mux.Lock()
        defer mux.Unlock()
  	n--
  }
}

I'll put a PR up just to show you how I've tried it to implement the ConnectionClosedCallback, but I am of course open to suggestions!