[SPARK-2525][SQL] Remove as many compilation warning messages as possible in Spark SQL

JIRA: https://issues.apache.org/jira/browse/SPARK-2525.

Author: Yin Huai <huai@cse.ohio-state.edu>

Closes #1444 from yhuai/SPARK-2517 and squashes the following commits:

edbac3f [Yin Huai] Removed some compiler type erasure warnings.

(cherry picked from commit df95d82da7c76c074fd4064f7c870d55d99e0d8e)
Signed-off-by: Reynold Xin <rxin@apache.org>
[SPARK-2518][SQL] Fix foldability of Substring expression.

This is a follow-up of #1428.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #1432 from ueshin/issues/SPARK-2518 and squashes the following commits:

37d1ace [Takuya UESHIN] Fix foldability of Substring expression.

(cherry picked from commit cc965eea510397642830acb21f61127b68c098d6)
Signed-off-by: Reynold Xin <rxin@apache.org>
SPARK-1097: Do not introduce deadlock while fixing concurrency bug

We recently added this lock on 'conf' in order to prevent concurrent creation. However, it turns out that this can introduce a deadlock because Hadoop also synchronizes on the Configuration objects when creating new Configurations (and they do so via a static REGISTRY which contains all created Configurations).

This fix forces all Spark initialization of Configuration objects to occur serially by using a static lock that we control, and thus also prevents introducing the deadlock.

Author: Aaron Davidson <aaron@databricks.com>

Closes #1409 from aarondav/1054 and squashes the following commits:

7d1b769 [Aaron Davidson] SPARK-1097: Do not introduce deadlock while fixing concurrency bug
(cherry picked from commit 8867cd0bc2961fefed84901b8b14e9676ae6ab18)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>
[SPARK-2154] Schedule next Driver when one completes (standalone mode)

Author: Aaron Davidson <aaron@databricks.com>

Closes #1405 from aarondav/2154 and squashes the following commits:

24e9ef9 [Aaron Davidson] [SPARK-2154] Schedule next Driver when one completes (standalone mode)
(cherry picked from commit 9c249743eaabe5fc8d961c7aa581cc0197f6e950)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>