raft: remove unused return
Merge pull request #1084 from unihorn/126

raft: remove unused return
etcdhttp: check method for every endpoint, add tests
Merge pull request #1069 from jonboulle/methods

etcdhttp: check method for every endpoint, add tests
etcdserver: introduce Server interface

This changes etcdserver.Server to an interface, with the former Server
(now "EtcdServer") 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.
etcdserver/etcdhttp: add test coverage for serveRaft