Rename Host to Proxy

[doyoubi]

At the first time, I think we will only deploy one proxy per host. So host and proxy are the same and are used interchangeably in the codes and API.
Now to support some clients and redis cluster proxies which do not support AUTH command for the backend clusters, we need to deploy multiple proxies in the same machine to support multiple tenants.
I have changed the API in #21.
Later we need to change the host in code to proxy.

[doyoubi]

Also rename db_name to cluster_name and use
#[serde(alias = "fancy_square", alias = "KUBE")]
for compability.