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

[zsxwing]

Reuse Text in saveAsTextFile to reduce GC.

/cc @rxin

[SparkQA]

Test build #24704 has started for PR 3762 at commit 59f03eb.

• This patch merges cleanly.

[srowen]

I think it's a small but OK optimization. Hadoop won't save the Text object itself, so it's safe, here in the 'save' method.

[SparkQA]

Test build #24704 has finished for PR 3762 at commit 59f03eb.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/24704/
Test PASSed.

[sryza]

Great idea, LGTM

[rxin]

LGTM. Merging in master. Thanks.