raft: log start and stop of the node

The log is like this:
```
2014/12/08 13:48:29 raft.node: 924e2e83e93f2560 [leader: 0, softState: {Lead:0 RaftState:StateFollower Nodes:[]}, hardState: {Term:3 Vote:0 Commit:32 XXX_unrecognized:[]}] starts running
```
for #1877