Versioning or unique object identifiers in storage

## Feature Request

I want to synchronize my local files with different cloud storages. So, for this I would like to have some mechanism to check if a file in the cloud differs from my file. Unfortunately, I could not find any appropriate feature in libcloud.

I propose using the same hash method for all storage drivers and make it public, so anyone can check if file is the same in a local system and in any cloud.

However, this approach doesn't allow to determine where the file was changed, so, alternatively, we can use modify_time from `os.stat(file).st_mtime` or, maybe, you have a better idea.

首先检查 libcloud 的 storage-driver 接口以及远程文件元数据的公开方式，然后将提议的通用哈希与 os.stat(file).st_mtime 进行比较。该 issue 未指定任何文件或测试；要完成这项工作，需要一个各 driver 之间达成共识的标识符或变更检测契约，以及一个明确的公共 API。

由索引模型根据 Issue 内容生成。