Changes

PackBitmapIndexBuilder.StoredEntry: add getter for objectId

If you only have access to the PackBitmapIndexBuilder there is no way to
get the ObjectId from a StoredEntry instance without also having access
to a reverse pack index. The StoredEntry can provide the idxPosition,
but the PackBitmapIndexBuilder's getObject method requires a
ridxPosition in order to find an ObjectId.

Providing an ObjectId from the StoredEntry gives this information
directly and also allows a caller to get the ridxPosition if desired by
calling PackBitmapIndexBuilder.findPosition(objectId) without needing
an index object.

This closes the operations of the PackBitmapIndexBuilder such that any
method can be called by using information provided by the other methods.

Change-Id: I5a11479b9635cd6b5e7aaff2f862cd41069ac469