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
Yes, it has a subset feature of EvalNode. 
Given a conjunctive form of query predicates, we need to disjoint them to find an alternative access path using a proper index; first, we find a proper index with the columns involved in predicates. After that, we estimate the selectivity of those predicates.

As you know, the graph of EvalNodes can represent complex and conjunctive predicates. So, in order to find the index scan path, we need to decompose the EvalNode graph into the several disjoint predicates. 

Currently, the BST index supports only the equality predicate, I used Predicate and SimplePredicate classes as a temporal solution to represent a simple predicate which has a form of 'column = value' or 'value = column'. 

However, we need to devise a better representation for other types of predicates such as range predicate.