adds ability to control what values (if any) for avg_rt will be sent if ...

...no requests were done, fixes #212

I did basic testing and it looks ok, please review

Usage:

```--carbon-idle-avg none``` - don't push any avg_rt value if no requests were made
```--carbon-idle-avg last``` - use last computed avg_rt value (default)
```--carbon-idle-avg zero``` - push 0 if no requests were made