[SPARK-21745][SQL] Refactor ColumnVector hierarchy to make ColumnVector read-only and to introduce WritableColumnVector.

## What changes were proposed in this pull request?

This is a refactoring of `ColumnVector` hierarchy and related classes.

1. make `ColumnVector` read-only
2. introduce `WritableColumnVector` with write interface
3. remove `ReadOnlyColumnVector`

## How was this patch tested?

Existing tests.

Author: Takuya UESHIN <ueshin@databricks.com>

Closes #18958 from ueshin/issues/SPARK-21745.