In DatabaseImpl::dropCollectionEvenIfSystem(), we currently rename the collection to a <db>.system.drop.* namespace. With drop-pending ident support, we will instead invoke _finishDropCollection() to remove the collection from the catalog.

Subsequently, the KV storage engine will remove the collection from the KVCatalog. Rather than dropping the ident, the KVDatabaseCatalogEntryBase::RemoveCollectionChange will register the ident with the KVDropPendingIdentReaper using the commit timestamp from the RecoveryUnit.

With the new Timestamp notifier introduced in SERVER-38128, it is now possible for the KVStorageEngine to set up a listener for changes to either the oldest or checkpoint timestamps together with a KVDropPendingIdentReaper so that drop-pending idents with commit timestamps that precede the oldest timestamp can be safely removed.