[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 Shark). Also, before this PR, `NullableColumnBuilder` copies the whole buffer to add the null positions section, and then `CompressibleColumnBuilder` copies and compresses the buffer again. In this PR the first buffer copy is eliminated to reduce memory consumption.