Fix a silly bug for unlimited pools.

Unlimited quantity -1 is always less than consumed, so if a pool's quantity
changed to unlimited, we would try to revoke every outstanding entitlement.
1150234: Fix refresh pools instance does not yet exist in db error.

Previous attempted fix just avoided the problem by not updating attributes if
the pool was flagged for deletion. However this missed the fact that the
scenario can also occur if the pool attributes change, and entitlements still
need to get revoked. (overconsumption)

Seems a flush is recommended to make sure the modified attributes are created
in the db, after which we can successfully lock the pool and revoke any
entitlements required.

PR also fixes a nasty discovery with the "isOverflowing" check, which was not
accounting for unlimited pools. (quantity -1) As it stands now it appears if an
unlimited pool was changed in any way during refresh, we'd revoke every
entitlement it has.
Merge pull request #753 from candlepin/dgoodwin/row-doesnt-exist

Fix entity does not exist as row in database error