According to http://godoc.org/fmt#Scan, if scan number is less than the
number of arguments, err will report why. So we don't need to handle
this error case.
1. etcd fatals if there is critical error in the system and operator should
do something for it
2. etcd panics if there happens something unexpected, and it should be
reported to us to debug.
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.
