[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.