[SPARK-16104][SQL] Do not creaate CSV writer object for every flush when writing

[HyukjinKwon]

What changes were proposed in this pull request?

This PR let CsvWriter object is not created for each time but able to be reused. This way was taken after from JSON data source.

Original CsvWriter was being created for each row but it was enhanced in #13229. However, it still creates CsvWriter object for each flush() in LineCsvWriter. It seems it does not have to close the object and re-create this for every flush.

It follows the original logic as it is but CsvWriter is reused by reseting CharArrayWriter.

How was this patch tested?

Existing tests should cover this.

[HyukjinKwon]

Please let me organise this in a separate class at the PR for refactoring this.

[SparkQA]

Test build #60932 has finished for PR 13809 at commit a62a26f.

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

[HyukjinKwon]

Hm.. I will try to minimise the changes here.

[HyukjinKwon]

Hi @davies, it seems the past change in your PR is related with this. Could you please take a look?

[SparkQA]

Test build #60986 has finished for PR 13809 at commit a96d547.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #60985 has finished for PR 13809 at commit 61dfef8.

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

[SparkQA]

Test build #60988 has finished for PR 13809 at commit 882a24f.

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

[davies]

LGTM,
Merging this into master, thanks!