Set a sensible default retention/aggregation policy; enable auto-resize on carbon start.

This PR improves our Graphite installations to keep a healthy amount of data for long periods:

- 10s resolution for 10 days (collectd/statsd update interval), so we don't lose resolution on most of our use cases.
- 5m for 400 days - reasonable accuracy for long-term data.
- 24hr for 20 years - so we don't throw away data.

In the process, ensure that we resize our whisper db files automatically if the configuration changes. A small delay on starting up carbon is better than writing to the wrong format db file.

Note that this could increase the size of your Graphite storage/whisper data considerably. It is worth doing a count of the number of whisper files in there, you will need 2.5MiB for each one after resize.

```
# size required in KB:
echo $(( $(find /srv/graphite/storage/whisper/ -name \*.wsp | wc -l) * 2500 ))
```