[SPARK-4918][Core] Reuse Text in saveAsTextFile

Reuse Text in saveAsTextFile to reduce GC.

/cc rxin

Author: zsxwing <zsxwing@gmail.com>

Closes #3762 from zsxwing/SPARK-4918 and squashes the following commits:

59f03eb [zsxwing] Reuse Text in saveAsTextFile
[SPARK-4915][YARN] Fix classname to be specified for external shuffle service.

Author: Tsuyoshi Ozawa <ozawa.tsuyoshi@lab.ntt.co.jp>

Closes #3757 from oza/SPARK-4915 and squashes the following commits:

3b0d6d6 [Tsuyoshi Ozawa] Fix classname to be specified for external shuffle service.
[SPARK-4870] Add spark version to driver log

Author: Zhang, Liye <liye.zhang@intel.com>

Closes #3717 from liyezhang556520/version2Log and squashes the following commits:

ccd30d7 [Zhang, Liye] delete log in sparkConf
330f70c [Zhang, Liye] move the log from SaprkConf to SparkContext
96dc115 [Zhang, Liye] remove curly brace
e833330 [Zhang, Liye] add spark version to driver log