FilterQuery (`filter([some_query])`) should implement DocSetProducer

There is a substantial practical reason for FilterQuery to implement DocSetProducer: the way FilterQuery works, it only consults the cache internally for the backing query; the cache is never directly consulted for the FilterQuery per se. As a consequence, when FilterQuery is used in a context that wants a DocSet, currently (without implementing DocSetProducer), a full clone of the cached DocSet must always be built via the Weight returned by FilterQuery.createWeight(). Implementing DocSetProducer allows to avoid superfluous DocSet creation.