raft: update unstable when calling stableTo with 0

It should update unstable in this case because it may happen that raft only writes entry
0 into stable storage.

for #1644 
It makes the test rather slow because etcdserver keeps getting new ready with entry 0 as entries and writing it to disk before the leader syncs the whole log to the follower. It may happen 10000+ times when increasing cluster size from 3 to 6.