[Spinoff from LUCENE-3390]
I think the approach in 3.x for handling un-valued docs, and making it
possible to specify how such docs are sorted, is better than the
solution we have in trunk.
I like that FC has a dedicated method to get the Bits for docs with field
– easy for apps to directly use. And I like that the
bits have their own entry in the FC.
One downside is that it's 2 passes to get values and valid bits, but
I think we can fix this by passing optional bool to FC.getXXX methods
indicating you want the bits, and the populate the FC entry for the
missing bits as well. (We can do that for 3.x and trunk). Then it's