[SPARK-22143][SQL] Fix memory leak in OffHeapColumnVector

## What changes were proposed in this pull request?
`WriteableColumnVector` does not close its child column vectors. This can create memory leaks for `OffHeapColumnVector` where we do not clean up the memory allocated by a vectors children. This can be especially bad for string columns (which uses a child byte column vector).

## How was this patch tested?
I have updated the existing tests to always use both on-heap and off-heap vectors. Testing and diagnoses was done locally.

Author: Herman van Hovell <hvanhovell@databricks.com>

Closes #19367 from hvanhovell/SPARK-22143.