Refine the statistics estimation for the limit and aggregate operator

[yahoNanJing]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Statistics is very useful for making intelligent decisions, especially for choosing proper join implementation. Therefore, it's better to introduce it into each plan operator as much as possible.

Describe the solution you'd like

• For the limit operator, the output row count is surely not larger than its limited number;
• For the aggregate operator, the output row count is surely not larger than its input row count.

Describe alternatives you've considered

Additional context