Add raft msg denied

1. Fix genproto.sh

2. Add denied field to indicate a rejection

- Prepare for making index int64 -> uint64
- Prepare for fixing msgResp duplication: 

If leader sends two identical msgApps to a follower, the follower might send back two identical msgAppResp.

If the follower sends back two denial msgAppResps, leader will decrease the next field of progress twice. 

This is incorrect. We expect the leader only decrease next field once. We reserve the index filed to indicate which index the follower actually rejected to.
see line 4 - the idea was that you just run `scripts/genproto.sh`. That way the `GOPATH` that gets set up is in the top-level `gopath/` directory which is in .gitignore..