1002946: Fix entitlement dates not being updated when pool dates change.

Caused issues where sub-pool dates looked correct, but consumer status
was inaccurate because calculation was done using entitlement dates
rather than the pool.

No need for duplication of this data, so drop the entitlement date
columns and use the pool instead.

Entitlement will still report a start/end date to maintain API
compatability, it just comes from the pool now.
Merge pull request #369 from candlepin/dgoodwin/drop-ent-dates

1002946: Fix entitlement dates not being updated when pool dates change.