SERVER-79480 involves changing the rollover behavior of buckets to accommodate for the fact that there will be multiple open buckets. This is a split off ticket from that ticket that handles moving the code that handles determining rollover action from insertIntoBucket into useBucket - this will change the contract for useBucket to return a bucket with no RolloverAction that can be inserted into by insertIntoBucket safely, and rollover buckets if there are no such buckets. It will essentially also collapse the functionality of useAlternateBucket into useBucket.