TAJO-838: Improve query planner to utilize index 

In this work, the previous work is extended to support indexes for multiple columns or even expressions. Even though we don't provide any methods to estimate the query selectivity, users can use the index scan by configuring the session variables. Finally, I have added more unit tests including performing join, aggregation, sort after index scans.

As summarized below, some necessary features are still lacked in this work.
However, IMHO, this work is ready to be merged into master branch as a prototype of query execution using index. Please leave any comments.
Following features should be added.
* Query optimization based on selectivity estimation
* Support index for partitioned tables
Thanks for the variable comment. I'll refactor this part according to your comment.