[SPARK-2650][SQL] Try to partially fix SPARK-2650 by adjusting initial buffer...

[SPARK-2650][SQL] Try to partially fix SPARK-2650 by adjusting initial buffer size and reducing memory allocation

JIRA issue: [SPARK-2650](https://issues.apache.org/jira/browse/SPARK-2650)

Please refer to [comments](https://issues.apache.org/jira/browse/SPARK-2650?focusedCommentId=14084397&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14084397) of SPARK-2650 for some other details.

This PR adjusts the initial in-memory columnar buffer size to 1MB, same as the default value of Shark's `shark.column.partitionSize.mb` property when running in local mode. Will add Shark style partition size estimation in another PR.

Also, before this PR, `NullableColumnBuilder` copies the whole buffer to add the null positions section, and then `CompressibleColumnBuilder` copies and compresses the buffer again, even if compression is disabled (`PassThrough` compression scheme is used to disable compression). In this PR the first buffer copy is eliminated to reduce memory consumption.

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

Closes #1769 from liancheng/spark-2650 and squashes the following commits:

88a042e [Cheng Lian] Fixed method visibility and removed dead code
001f2e5 [Cheng Lian] Try fixing SPARK-2650 by adjusting initial buffer size and reducing memory allocation