raft: set raft.Commit too when setting raftLog.committed

This helps to apply init entries when advance before any raft.Commit refresh.

Or it cannot commit to the latest commit because raft.Commit is set wrong. (https://github.com/coreos/etcd/blob/master/raft/node.go#L315)

the fix on snapshot test will be improved after i fix snapi.
why do we remove this?