[SPARK-16862] Configurable buffer size in `UnsafeSorterSpillReader`

## What changes were proposed in this pull request?

Jira: https://issues.apache.org/jira/browse/SPARK-16862

`BufferedInputStream` used in `UnsafeSorterSpillReader` uses the default 8k buffer to read data off disk. This PR makes it configurable to improve on disk reads. I have made the default value to be 1 MB as with that value I observed improved performance.

## How was this patch tested?

I am relying on the existing unit tests.

## Performance

After deploying this change to prod and setting the config to 1 mb, there was a 12% reduction in the CPU time and 19.5% reduction in CPU reservation time.

Author: Tejas Patil <tejasp@fb.com>

Closes #14726 from tejasapatil/spill_buffer_2.