Deferred access for large objects

[GoogleCodeExporter]

When a large objects are read from an input along with a bunch of other
fields (eg metadata about the object) and then the metadata is processed by
several expensive operations, it sometime make sense to defer the retrieval
of the large object and go back to the input to fetch the large object
after processing.  We could do few things here:

1. support large object handles that are fetched later.
2. a cost-based rewrite that considers early vs late fetching of the large
object.

At a minimum, we should do what is necessary to allow a user to do this
manually without a lot of effort.

Original issue reported on code.google.com by [email protected] on 20 Mar 2009 at 9:29