Add/increase severity of warning in documentation of groupBy()

groupBy()/groupByKey() is notorious for being a very convenient API that can lead to poor performance when used incorrectly.

This PR just makes it clear that users should be cautious not to rely on this API when they really want a different (more performant) one, such as reduceByKey().

(Note that one source of confusion is the name; this groupBy() is not the same as a SQL GROUP-BY, which is used for aggregation and is more similar in nature to Spark's reduceByKey().)

Author: Aaron Davidson <aaron@databricks.com>

Closes #1380 from aarondav/warning and squashes the following commits:

f60da39 [Aaron Davidson] Give better advice
d0afb68 [Aaron Davidson] Add/increase severity of warning in documentation of groupBy()
(cherry picked from commit a2aa7bebae31e1e7ec23d31aaa436283743b283b)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>
[SPARK-2485][SQL] Lock usage of hive client.

Author: Michael Armbrust <michael@databricks.com>

Closes #1412 from marmbrus/lockHiveClient and squashes the following commits:

4bc9d5a [Michael Armbrust] protected[hive]
22e9177 [Michael Armbrust] Add comments.
7aa8554 [Michael Armbrust] Don't lock on hive's object.
a6edc5f [Michael Armbrust] Lock usage of hive client.

(cherry picked from commit c7c7ac83392b10abb011e6aead1bf92e7c73695e)
Signed-off-by: Aaron Davidson <aaron@databricks.com>