simulate fs

[wangrunji0408]

Now we only have a very simple file system simulator for the raft use. We need to improve that.

[m-mueller678]

I need this, and I am happy to implement it. As the current API is very minimal, do you have any input on what the API should look like?

I am also unsure on what the semantics of concurrent operations should be. The simplest approach would be to linearize everything in the background, though that probably rules out a lot of weird concurrency behavior that you might observe on a real filesystem.