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.
I think it is a little weird that this belongs to interface. This thread is just for discussion.
It is used for etcdhttp to put message received into raft state machine, but the other funcs related to state machine (`Save`, `Send`) are set from very beginning.