[SPARK-12968][SQL] Implement command to set current database

[viirya]

JIRA: https://issues.apache.org/jira/browse/SPARK-12968

Implement command to set current database.

[rxin]

cc @hvanhovell for review.

[SparkQA]

Test build #50071 has finished for PR 10916 at commit 46737b5.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • case class SetDatabaseCommand(databaseName: String) extends RunnableCommand

[viirya]

retest this please.

[SparkQA]

Test build #50079 has finished for PR 10916 at commit 43beb4b.

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

[SparkQA]

Test build #50088 has finished for PR 10916 at commit ee237e4.

• This patch fails Scala style tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #50090 has finished for PR 10916 at commit bffae43.

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

[SparkQA]

Test build #50104 has finished for PR 10916 at commit 9df380f.

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

[hvanhovell]

Maybe a dumb idea: we could return Ok...

[viirya]

Return OK will break hive compatibility test. I've tried in previous commits.

[rxin]

@viirya can you remove ur last commit? I can merge this. I'm redesigning catalog right now and we don't need to add that no-op commit here.

[viirya]

@rxin ok. I will update this later (about 30 mins) when I am back to desk.

[SparkQA]

Test build #50337 has finished for PR 10916 at commit 87c27ef.

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

[viirya]

@rxin I've reverted last commit.

[rxin]

Thanks - I've merged this.

[SparkQA]

Test build #50350 has finished for PR 10916 at commit ff5508b.

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