SPARK-1860: Do not cleanup application work/ directories by default

This causes an unrecoverable error for applications that are running for longer
than 7 days that have jars added to the SparkContext, as the jars are cleaned up
even though the application is still running.

Author: Aaron Davidson <aaron@databricks.com>

Closes #800 from aarondav/shitty-defaults and squashes the following commits:

a573fbb [Aaron Davidson] SPARK-1860: Do not cleanup application work/ directories by default
[Spark-1461] Deferred Expression Evaluation (short-circuit evaluation)

This patch unify the foldable & nullable interface for Expression.
1) Deterministic-less UDF (like Rand()) can not be folded.
2) Short-circut will significantly improves the performance in Expression Evaluation, however, the stateful UDF should not be ignored in a short-circuit evaluation(e.g. in expression: col1 > 0 and row_sequence() < 1000, row_sequence() can not be ignored even if col1 > 0 is false)

I brought an concept of DeferredObject from Hive, which has 2 kinds of children classes (EagerResult / DeferredResult), the former requires triggering the evaluation before it's created, while the later trigger the evaluation when first called its get() method.

Author: Cheng Hao <hao.cheng@intel.com>

Closes #446 from chenghao-intel/expression_deferred_evaluation and squashes the following commits:

d2729de [Cheng Hao] Fix the codestyle issues
a08f09c [Cheng Hao] fix bug in or/and short-circuit evaluation
af2236b [Cheng Hao] revert the short-circuit expression evaluation for IF
b7861d2 [Cheng Hao] Add Support for Deferred Expression Evaluation