PackObjectSizeIndex: Read all bytes and use the byte[] directly

PackObjectSizeIndex: Read all bytes and use the byte[] directly

The parser reads N integers one by one from the stream, assuming the
InputStream does some ahead reading from storage. We see some very
slow loading of indexes and suspect that this preemptive reading is
not happening. The slow loading can be reproduced in clones, and it
produces higher latencies and locks many threads waiting for the
loading.

Read the whole array from storage in one shot to avoid many small IO
reads. Work directly on the resulting byte[], so there is no need of a
second copy to cast to int/long.

This is how other indexes, like primary or commit graph, work.

Change-Id: I60058606e2c457f60aa4646a1f10ae7b28ce34c2