Implement version control

[fern4lvarez]

Version control will be the way piladb persists its contents: instead of writing data into disk, it will decode all atomic operations into a binary log, that will contain the history of the piladb server. This will make possible to recover the state of a piladb instance from a file in order to implement #14, and also to share the state with other instances in a distributed fashion, so we can achieve #13.