optimize DocList creation by skipping sort for sort-irrelevant cases

When rows=0, and for MatchAllDocsQuery and ConstantScoreQuery (and possibly others?), it is possible for SolrIndexSearcher.getDocListC(QueryResult, QueryCommand) to create a DocList directly from filterCache DocSets – similar to useFilterForSortedQuery, but without actually sorting.

This results in significant benefits for high-recall domains, including the common (and commonly-cached) use-case of q=*:* and fq, facets, etc.