add log abstraction to store records in disk

@xiangli-cmu 
This adds _log_ and _message_ abstraction.
I haven't thought about how to batch encoding yet. It had better be considered in snapshotting.

Just a follow up on this. According to [KAFKA doc](http://kafka.apache.org/documentation.html#persistence), "As a result of these factors using the filesystem and relying on pagecache is superior to maintaining an in-memory cache or other structure". It sounds OS page-cache reliance is a better option. And thanks for the discussion here.