PackIndex: expose the position of an object-id in the index

PackIndex: expose the position of an object-id in the index

The primary index returns the offset in the pack for an
objectId. Internally it keeps the object-ids in lexicographical order,
but doesn't expose an API to find the position of an object-id in that
list. This is needed for the object-size index, that we want to store
as "position-in-idx, size".

Add a #findPosition(object-id) method to the PackIndex interface to
know where an object-id sits in the ordered list of ids in the pack.

Note that this index position is over the list of ordered object-ids,
while reverse-index position is over the list of objects in packed
order.

Change-Id: I89fa146599e347a26d3012d3477d7f5bbbda7ba4