Improve ARAX's ranking algorithm

[chunyuma]

This is a super issue for "Better Ranking". I split it into the following few sub-tasks:

• Adjust weights in the ranker. See the sub-task in Adjust weights in the ranker #2300.
• Add a function in ranker to give higher weight based on the number of edges. See the sub-task in Add a function in ranker to give higher weight based on the number of edges #2301.
• Gather examples and write a few test cases for evaluation. See the sub-task in Gather examples and write a few test cases for ranker evaluation #2302.
• Final evaluation after deployment.