Fix continuous loop of connect attempts

Sometimes zookeeper just drops connection on invalid session data,
we prefer to drop session and start from scratch when that event
occurs instead of dropping into loop of connect/disconnect attempts.

This can be emulated by stopping zookeeper, cleaning it's database and
starting it again. But we experience same bug in production without
cleaning zookeeper, but in case of network partition (not every partition
event).

zk/conn.go

@@ -366,7 +366,14 @@ func (c *Conn) authenticate() error {
 	// package length
 	_, err = io.ReadFull(c.conn, buf[:4])
 	if err != nil {
-		return err
+		// Sometimes zookeeper just drops connection on invalid session data,
+		// we prefer to drop session and start from scratch when that event
+		// occurs instead of dropping into loop of connect/disconnect attempts
+		c.sessionID = 0
+		c.passwd = emptyPassword
+		c.lastZxid = 0
+		c.setState(StateExpired)
+		return ErrSessionExpired
 	}
 
 	blen := int(binary.BigEndian.Uint32(buf[:4]))