Use a bucket sort for PackReverseIndex.

Use a bucket sort for PackReverseIndex.

Previously it took 1200ms to create a reverse index (sorted by offset).
Using a simple bucket sort algorithm, that time is reduced to 450ms.
The bucket index into the offset array is kept, in order to decrease
the binary search window.

Don't keep a copy of the offsets. Instead, use nth position
to lookup the offset in the PackIndex.

Change-Id: If51ab76752622e04a4430d9a14db95ad02f5329d