Improve handling of searches for _summary=count

The FHIR search API provides a mechanism for requesting less that a server provides less that the full resources requested for a search using the _summary parameter. Details of this parameter are documented here.

For the most part, HAPI handles this for us and we do not need to do anything with this parameter. However, the one case that is less than ideal is when the `_summary=count` is specified. This is because we use this IBundleProvider to provide the resources that match a search. The problem is that HAPI will try to get all of the resources that a bundle might provide when the user is only interested in returning the count of resources.

Where the request is for _summary=count, we should ensure we only do the initial search for the matching resource UUIDs and not perform the actual search at all.