In the delete code path, we currently reserve oplog slots for retryable findAndModify, which takes up replication resources, before calling OpObserver::aboutToDelete(), which does not really require the oplog resources and needs to notify numerous OpObserver implementations before completing.

The oplog slots are needed for OpObserver::onDelete() to write the oplog entries so it would be beneficial to defer reserving the oplog slots until aboutToDelete() returnes.