Support store metadata with rpc service not with db or KV directly

[rongfengliang]

What would you like to be added:

currently juicefs metadata storage use DB or KV storage. this is not good for security or management in enterprise

Why is this needed:

can we use RPC service or other for good management I know with RPC or others may be difficult (transaction is one most important thing )

[davies]

If we create a RPC service for JuiceFS, then we should provide the way and tools for management, those will be similar to that provided by existing database (Redis and other SQL databases). So, I think reuse the existing tools to manage DB or KV is better than re-build them.

JuiceFS serves as a bottom infrastructure tool, we can build some tool to manage it, to avoid expose the database directly to end users.

Adding a RPC service will increase latency to meta operation, which is critical to JuiceFS, we'd like to avoid that as much as possible.

[rongfengliang]

@davies Yes, I agree, Plan to add feature like juicefs pro so we can use token to manage backend metadata storage