etcdserver: introduce Server interface

This changes etcdserver.Server to an interface, with the former Server
(now "server") becoming the canonical/production implementation. This will
facilitate better testing of the http server et al with mock implementations
of the interface. It also more clearly defines the boundary for users of the
Server. A NewServer convenience function is added to create the Server with
some default/production values.
@philips That's the same feeling as mine.
What @xiangli-cmu and I do before is to split etcdhttp into two structs: one is for /raft endpoint, and the other is for /v2/keys. The first one transfers messages to raft.Node, and is not exposed in Server interface.