SolrNamedSource: New registry class, use in PrimarySearch + YodaQA pipeline

It dramatically simplifies Solr data source setup and enwiki/guten
switching, while also allowing for easier separation of search
and retrieval, which is just around the corner.
PrimarySearch: Split the CAS multiplier off to ResultGenerator

Now, we will be able to create multiple PrimarySearch annotators,
e.g. another for a "title-in-clue" search.  We will need to run
multiple parallel CAS multipliers soon anyway (e.g. for passages
from document titles), but this separation is still useful to
deduplicate results between whole-document searches.