remove the closeCallback from the session

The closeCallback was run when a session was closed, i.e. after the run
loop of the session stopped. Instead of explicitely calling this callback
from the session, the caller of session.run() can just execute the code
after session.run() returns.

client.go

@@ -244,18 +244,27 @@ func (c *client) createNewSession(negotiatedVersions []protocol.VersionNumber) e
 		c.version,
 		c.connectionID,
 		c.config.TLSConfig,
-		c.closeCallback,
 		c.cryptoChangeCallback,
-		negotiatedVersions)
+		negotiatedVersions,
+	)
 	if err != nil {
 		return err
 	}
 
-	go c.session.run()
+	go func() {
+		// session.run() returns as soon as the session is closed
+		err := c.session.run()
+		if err == errCloseSessionForNewVersion {
+			return
+		}
+
+		c.mutex.Lock()
+		c.listenErr = err
+		c.connStateChangeOrErrCond.Signal()
+		c.mutex.Unlock()
+
+		utils.Infof("Connection %x closed.", c.connectionID)
+		c.conn.Close()
+	}()
 	return nil
 }
-
-func (c *client) closeCallback(_ protocol.ConnectionID) {
-	utils.Infof("Connection %x closed.", c.connectionID)
-	c.conn.Close()
-}