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
I think that the role of ```AccessPathRewriterContext``` moved into SessionVars. AccessPathRewriterContext includes a flag ```enableIndex``` and a float variable ```selectivityThreshold```. SessionVars can cover its role. 

Also, AccessPathRewriterContext includes one rewrite rule which is performed according to two variables of AccessPathRewriterContext. For that, it would be better if you change two methods ```isEligible``` and ```rewrite```  ```isEligible(OverridenConf, LogicalPlan)``` and ```rewrite(OverriddenConf, LogicalPlan)```. Because OverriddenConf which is a super class of ```QueryContext``` which includes all variables in SessionVars, two methods will work according to ```enableIndex``` and ```selectivityThreshold``` given by users.