[SPARK-15822] [SQL] Prevent byte array backed classes from referencing freed memory

## What changes were proposed in this pull request?
`UTF8String` and all `Unsafe*` classes are backed by either on-heap or off-heap byte arrays. The code generated version `SortMergeJoin` buffers the left hand side join keys during iteration. This was actually problematic in off-heap mode when one of the keys is a `UTF8String` (or any other 'Unsafe*` object) and the left hand side iterator was exhausted (and released its memory); the buffered keys would reference freed memory. This causes Seg-faults and all kinds of other undefined behavior when we would use one these buffered keys.

This PR fixes this problem by creating copies of the buffered variables. I have added a general method to the `CodeGenerator` for this. I have checked all places in which this could happen, and only `SortMergeJoin` had this problem.

This PR is largely based on the work of robbinspg and he should be credited for this.

closes https://github.com/apache/spark/pull/13707

## How was this patch tested?
Manually tested on problematic workloads.

Author: Pete Robbins <robbinspg@gmail.com>
Author: Herman van Hovell <hvanhovell@databricks.com>

Closes #13723 from hvanhovell/SPARK-15822-2.