Make server APIs protocol agnostic

[akiradeveloper]

Suppose we build a REST server that use Raft consensus algorithm to share the state. As of 0.5.0, it is only possible by building an REST server that proxies http requests to lol server that is listening to gRPC requests.

While RPCs like append-entries and request-votes are inter-node and they needs optimization like gRPC streaming, request-apply and others are for simple client-interactions and the latter can be protocol agnostic so we can statically link other protocol layer on top of the APIs.