or Jenkins will complain about no Apache header in checkpoint files. tdas rxin

Author: Xiangrui Meng <meng@databricks.com>

Closes #2046 from mengxr/tmp-checkpoint and squashes the following commits:

0d3ec73 [Xiangrui Meng] remove ssc.stop
9797843 [Xiangrui Meng] change checkpointDir to lazy val
89964ab [Xiangrui Meng] use temp folder for checkpoint