Pluggable storage engine design

In this jira, we discuss how to make Cassandra's storage engine to be pluggable. We will discuss the scope, expectation, and guideline for this project, as well as a detailed design so that we can create sub tasks for each small project.

Here is a design doc we are currently working on: https://docs.google.com/document/d/1suZlvhzgB6NIyBNpM9nxoHxz_Ri7qAm-UEO8v8AIFsc