Need a more clear behavior about how to load needed statistics

## Enhancement

Some facts of the current TiDB:
- We've introduced `ANALYZE TABLE PREDICATE COLUMNS` and `ANALYZE TABLE COLUMNS` to collect parts of table columns to improve the speed of analyze and optimize the collected statistics' memory usage.
- When we use this two ways to collect statistics, the stats of one column that is not a predicate or not specified in `COLUMN_LIST` will not be deleted.
- If one SQL marks that column as stats needed, we'll load its outdated stats into memory.
- In many cases, the loading procedure will not be a problem. But if those columns are quite large. It will affects the performance of sync load.
- And there'll be the same problem for change value of the variable `tidb_analyze_skip_column_types`