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.
The change is here because before this PR, the initial committed entries will be sent through ready always before someone becomes the leader. So after Campaign, the etcdserver loop still can get the initial commits. The PR fixes the redundant committed entries through ready, so it needs to fix this place now.