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 
It is written here:
https://github.com/coreos/etcd/pull/1045/files#diff-a1d8c721ef8c6adb9070503b04c0d03eR209
index is given by the user.
I will do removal of index argument in another PR