Provide a means to force SearchHandler's distributed logic with 1 shard

SearchHandler (with its chain of SearchComponents) either operates as a coordinator of distributed search, or processes locally.  If there is only one shard, the latter will always happen, but it can be advantageous for authors of custom SearchComponents or SearchHandler subclasses to force the distributed search logic.  A simple example is to simplify the development / maintenance of authoring certain components.  Let's provide an internal API to force that.