Prevent peaks in traffic graph when restarting tor daemon.

For not losing current data, do:
cd /var/lib/munin/<DOMAIN>
mv <HOST>-tor_traffic-read-c.rrd <HOST>-tor_traffic-read-d.rrd
mv <HOST>-tor_traffic-written-c.rrd <HOST>-tor_traffic-written-d.rrd
rrdtool <HOST>-tor_traffic-read-d.rrd --data-source-type DERIVE
rrdtool <HOST>-tor_traffic-written-d.rrd --data-source-type DERIVE

![image](https://cloud.githubusercontent.com/assets/1649603/5423715/d7a9abe4-82cb-11e4-95e5-ead415ffda77.png)