[SPARK-13100] [SQL] improving the performance of stringToDate method in DateTimeUtils.scala

[yangw1234]

In jdk1.7 TimeZone.getTimeZone() is synchronized, so use an instance variable to hold an GMT TimeZone object instead of instantiate it every time.

[JoshRosen]

Jenkins, this is ok to test.

[SparkQA]

Test build #50444 has finished for PR 10994 at commit 19defc9.

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

[srowen]

LGTM. Are the other such instances in the code? Best to look for these all at once

[yangw1234]

@srowen No, just that one in this file.

[rxin]

Thanks - merging this in.

[carsonwang]

I think we should also reusing a Calendar object in each thread. It is expensive to create java Calendar and Timezone objects each time the method is called. I noticed the same problem and I saw performance boost if reusing these objects. The duration of one stage was improved from 1.6 minutes to 1.2 minutes. @wangyang1992 @rxin

[rxin]

Can you submit a patch? Thanks.

[carsonwang]

OK. I will do that.