raft: write entry 0 into log

#1033 #1045 
I will do entry write check (including always write entry 0 as the first one),
and load last written entry in snapshot
in following PRs.
the log has `Entry{Term: 0, Index: 0}` if init (https://github.com/coreos/etcd/blob/master/raft/log.go#L29), or recover a fake entry with index offset when restoring snapshot (https://github.com/coreos/etcd/blob/master/raft/log.go#L178).
the check is to make sure that entries match with no snapshot or loaded snapshot well.
i do this check because i think they should match, and the check will make it safer.