Made sort_datum_stream_t use next_batch.
Made sort_datum_stream_t be non-templatized.
Made eager_datum_stream_t::count use next_batch.
Made concatmap_datum_stream_t not use source->next, because it may become protected soon.
Made some stream cache entry_t fields be const, made scoped_ptr_t<env_t> be passed by move semantics where appropriate, made stream cache not have dead code allowing unlimited chunk size.