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 first entry, which has index 0, has been committed on default.
`Commit: x` indicates that raft has committed x entries, whose index is 1, 2, ..., x.