[SPARK-9460] Avoid byte array allocation in StringPrefixComparator.

As of today, StringPrefixComparator converts the long values back to byte arrays in order to compare them. This patch optimizes this to compare the longs directly, rather than turning the longs into byte arrays and comparing them byte by byte (unsigned).

This only works on little-endian architecture right now.

Author: Reynold Xin <rxin@databricks.com>

Closes #7765 from rxin/SPARK-9460 and squashes the following commits:

e4908cc [Reynold Xin] Stricter randomized tests.
4c8d094 [Reynold Xin] [SPARK-9460] Avoid byte array allocation in StringPrefixComparator.