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.
i will add it back when we do the snapshot. 
Again, please do not worry about snapshot for now. 

It is easy for everybody who reviews the code if you just do one thing at one commit.