Add a fast_iter option to StoqlibResultSet

This will return named tuples instead of real objects, making loading the
data from the database *a lot* faster. Should be used in special ocasions
when only the properties are really necessary, and not the rest of the
object API.

Change-Id: I1c36b728fba3c3a2799f1c09b585d3e4c3b501b9