Refactoring

Clean start might require more than just setting the offsets to 0. In fact, setting it to zero will most likely not work, as the beginning of the offset may not start from zero. I guess the real question is what do you mean by cleanStart? Start from the very very beginning and ignore the log? If so, there's is option to get the offset for HEAD in kafka. Also, I'm not sure what we should do with the old content of the log in this case.