implement a POSIX-ish file system that only caches metadata locally.

[Xuanwo]

Steps

Implement following API, other API will implement later or never

• Lookup
• Forget
• GetAttr
• SetAttr
• Mkdir
• Unlink
• Rmdir
• Access
• Create
• Open
• Read
• Lseek
• Release
• Write
• Flush
• Fsync
• OpenDir
• ReadDir
• ReadDirPlus
• ReleaseDir
• FsyncDir
• StatFs