[SQL] Renamed ColumnStat to ColumnMetrics to avoid confusion between ColumnStats

Class names of these two are just too similar.

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

Closes #2189 from liancheng/column-metrics and squashes the following commits:

8bb3b21 [Cheng Lian] Renamed ColumnStat to ColumnMetrics to avoid confusion between ColumnStats
[SPARK-3300][SQL] No need to call clear() and shorten build()

The function `ensureFreeSpace` in object `ColumnBuilder` clears old buffer before copying its content to new buffer. This PR fixes it.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #2195 from viirya/fix_buffer_clear and squashes the following commits:

792f009 [Liang-Chi Hsieh] no need to call clear(). use flip() instead of calling limit(), position() and rewind().
df2169f [Liang-Chi Hsieh] should clean old buffer after copying its content.