Raise 20M TTL to 20 days.

We need to raise the TTL on 20m rollups to 20 days if we hope to return 1440 datapoints to the user.  The reason we'd like to return 1440 datapoints because we want ele to raise their maximum points returned to 1440, up from 1000.
Add comment explaining how TTL value is calculated
Merge pull request #370 from rackerlabs/stackedsax/raise_20m_ttl

Raise 20M TTL to 20 days.