[SPARK-15958] Make initial buffer size for the Sorter configurable

## What changes were proposed in this pull request?

Currently the initial buffer size in the sorter is hard coded inside the code and is too small for large workload. As a result, the sorter spends significant time expanding the buffer size and copying the data. It would be useful to have it configurable.

## How was this patch tested?

Tested by running a job on the cluster.

Author: Sital Kedia <skedia@fb.com>

Closes #13699 from sitalkedia/config_sort_buffer_upstream.