Cost-based Optimizer for Tajo

The current query optimizer is too simple and is not extensible. In addition, it does not support scalar and table subqueries.

So, I would like to propose the implementation of new optimizer which is extensible and is based on some cost model.

In my opinion, this work is not trivial and will take a very long time. Also, we will have many discussions on this issue. This is the first draft, and I would like to continue to improve the requirements and details.