RFC: proxy mode

This only implements the proxy-only mode, it does not support falling back from failling to join the active raft cluster.
One method that comes to my head is:
1. select a random host at the beginning
2. change to another one if last http connection fails

The thoughts of this approach:
keep using one host as long as possible. If we switch between different hosts too often, clients may see different result because some host may be outdated. And it is hard to debug because people don't give raftIndex to us.