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