Rework connection replacement.

We used to signal connection replacement by reusing the same connection
id.  This turned out to be racy, as we couldn't reliably discard old
answers after a connection id was refused.

We now use a new id for every new connection, and explicitly signal
stream replacement in the offer message.  This requires maintaining a
local id on the client side.

diskwriter/diskwriter.go

@@ -93,7 +93,7 @@ func (client *Client) Kick(id, user, message string) error {
 	return err
 }
 
-func (client *Client) PushConn(g *group.Group, id string, up conn.Up, tracks []conn.UpTrack) error {
+func (client *Client) PushConn(g *group.Group, id string, up conn.Up, tracks []conn.UpTrack, replace string) error {
 	if client.group != g {
 		return nil
 	}
@@ -105,6 +105,14 @@ func (client *Client) PushConn(g *group.Group, id string, up conn.Up, tracks []c
 		return errors.New("disk client is closed")
 	}
 
+	rp := client.down[replace]
+	if rp != nil {
+		rp.Close()
+		delete(client.down, replace)
+	} else {
+		log.Printf("Replacing unknown connection")
+	}
+
 	old := client.down[id]
 	if old != nil {
 		old.Close()