fix(cubestore): re-enable streaming for top-k

See parent commits for particular fixes and improvements to streaming.

This reverts commit f763044.

rust/cubestore/src/queryplanner/topk/plan.rs

@@ -307,7 +307,7 @@ pub fn plan_topk(
         sort,
         &node.snapshots,
         schema.clone(),
-        /*use_streaming*/ false, // TODO: enable streaming.
+        /*use_streaming*/ true,
         /*max_batch_rows*/ max(2 * node.limit, MIN_TOPK_STREAM_ROWS),
     )?;
     let agg_fun = node