Rip out most of multiraft.go and replace it with etcd/raft.

This is not yet usable and the storage layer is not yet implemented,
but it's enough to pass its own limited test suite.

Next steps will be to make changes in the etcd/raft package itself to expose an interface that does not require a goroutine per group as raft.Node does.

CC @cockroachdb/developers @xiangli-cmu 
I'm not sure yet. I think as long as the multiraft package exists I'd like to treat the fact that it's backed by etcd/raft as an implementation detail, so we need our own enum for this operation instead of just pointing to etcd/raft/raftpb.