CleanupGarbage deletes entities used to store items under given tombstones.

FinishPop completes one or more pop operations (for different sets) by submitting changes to datastore.

Item is what's stored in the set.

Listing is returned by 'List' call.

PopOp is an in-progress 'Pop' operation.

Set holds a set of Items and uses tombstones to achieve idempotency of Add.

Garbage is a list of tombstones to cleanup.