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/data/proxy_dmatrix.h

@@ -1,5 +1,5 @@
 /*!
- * Copyright 2020 XGBoost contributors
+ * Copyright 2020-2021 XGBoost contributors
  */
 #ifndef XGBOOST_DATA_PROXY_DMATRIX_H_
 #define XGBOOST_DATA_PROXY_DMATRIX_H_