casbin-raft implementation

[0x8f701]

I don't really like the idea of watchers because it's buggy and heavy, image when one enforcer AddPolicy, the others need to reload all the policies from DB.....

we should find other solutions.

See also: casbin/casbin#421

[0x8f701]

On the same machine we don't need multiple enforcers, we just need to wrap enforcer inside Arc<RwLock>.

We only need this when there are multiple enforcers on different machines and they need to synchronize each other.

[0x8f701]

Not sure if: https://github.com/railgun-rs/actix-raft helps

[0x8f701]

It seems that we can try to use actix-raft first, here an example project I found:

https://github.com/oronsh/raftor

[0x8f701]

Let's start the work from here: https://github.com/casbin-rs/casbin-raft and try to implement it step by step so that casbin scales well