Rewrite sparse dmatrix using callbacks. (#7092)

- Reduce dependency on dmlc parsers and provide an interface for users to load data by themselves.
- Remove use of threaded iterator and IO queue.
- Remove `page_size`.
- Make sure the number of pages in memory is bounded.
- Make sure the cache can not be violated.
- Provide an interface for internal algorithms to process data asynchronously.

src/common/quantile.cu

@@ -91,7 +91,6 @@ void PruneImpl(int device,
     }
 
     float w = back.rmin - front.rmax;
-    assert(w != 0);
     auto budget = static_cast<float>(d_out.size());
     assert(budget != 0);
     auto q = ((static_cast<float>(idx) * w) / (static_cast<float>(to) - 1.0f) + front.rmax);