[SPARK-1678][SPARK-1679] In-memory compression bug fix and made compression configurable, disabled by default

In-memory compression is now configurable in `SparkConf` by the `spark.sql.inMemoryCompression.enabled` property, and is disabled by default.

To help code review, the bug fix is in [the first commit](https://github.com/liancheng/spark/commit/d537a367edf0bf24d0b925cc58b21d805ccbc11f), compression configuration is in [the second one](https://github.com/liancheng/spark/commit/4ce09aa8aa820bbbbbaa0f3f084a6cff1d4e6195).

Author: Cheng Lian <lian.cs.zju@gmail.com>

Closes #608 from liancheng/spark-1678 and squashes the following commits:

66c3a8d [Cheng Lian] Renamed in-memory compression configuration key
f8fb3a0 [Cheng Lian] Added assertion for testing .hasNext of various decoder
4ce09aa [Cheng Lian] Made in-memory compression configurable via SparkConf
d537a36 [Cheng Lian] Fixed SPARK-1678
(cherry picked from commit 6d721c5f7131f7c9fe56c524133d70cb37f1222d)

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