rafthttp: better logging

Fix #1933 

Only print out the same and continuous error message once. 

Logging as follow 
```
2014/12/14 09:49:22 sender: error posting to 6e3bd23ae5f1eae0: dial tcp 127.0.0.1:7002: connection refused
2014/12/14 09:49:22 sender: the connection with 6e3bd23ae5f1eae0 becomes inactive
.....
2014/12/14 09:49:56 sender: the connection with 6e3bd23ae5f1eae0 becomes active
```

/cc @robszumski @kelseyhightower