wal: change wal filename format

Make raftIndex section to be expected raftIndex of next entry.

It makes filename more intuitive and straight-forward.

The commit also adds comments for filename format.

#1033 
i do not know why it matches the current implementation. I do not think this `The value is calculated by adding 1 to raftIndex of last entry written.` is true. and i cannot find out where you implement this.