Use simplest possible query for cron script

[SabreCat]

Before, the Group Plan scheduled script performed a MongoDB query filtering on both the existence of purchased.plan.customerId and an $or between several possible states of purchased.plan.dateTerminated that were valid for an active subscription. This operation was not performant, leading to a high number of scanned document keys and ultimately queries taking ten seconds or more to complete.

Now, we perform the minimal query for existence of purchased.plan.customerId, which should be quick to resolve using a sparse index. Then within the script, after retrieving each group, we check whether its purchased.plan.dateTerminated indicates an expired sub. If so, we clear out the purchased.plan.customerId and move on to the next, so we can rely on the main query to exclude that group in future runs.

[phillipthelen]

Looks good!