A HitCollector that accesses the document directly to get the values for a field. This HitCollector
has low memory usage (it iterates over the documents as neccessary), and is useful when you are doing
a collection where there are a limited number of documents, but a large number of terms in the entire index.
This method should cache up the FieldSelector, such that overhead will be as small as possible when
this method is called from the