[SPARK-2650][SQL] More precise initial buffer size estimation for in-memory column buffer

This is a follow up of #1880.

Since the row number within a single batch is known, we can estimate a much more precise initial buffer size when building an in-memory column buffer.

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

Closes #1901 from liancheng/precise-init-buffer-size and squashes the following commits:

d5501fa [Cheng Lian] More precise initial buffer size estimation for in-memory column buffer