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.
Current `Cut` could only happen on file that has entry 0 written. This is because it takes the give index to make the cut.
I will fix that in the following PR, and remove this restriction.