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 
Because I change the raftIndex section to be expected raftIndex of next entry.
The first entry with index 0 in the log is an empty one, and we don't write it into disk now.

We may write it into disk also later.