virtual file system / virtual object storage

[skyzh]

People are complaining (#120) that they don't understand why they cannot directly modify the file_list in the storage state. It's mainly because of the simulator code that don't create any real SST files.

In theory, we should be able to run all these tests and simulators in memory without creating any actual files on the disk. Implement virtual file system can also help make tests faster.

[baobaomaomeng]

YES,I understand it now.But i think it should be mentioned in mini-lsm book,other it will confused