When initializing a collection object, we sometimes need to get the index ident from the durable catalog.

However, this read from the durable catalog is redundant, since we've already read the catalog entry from the durable catalog before calling this function.

We should find a way to remove the additional read from the durable catalog during CollectionImpl::initFromExisting by passing the necessary information from the durable catalog through the call site.

The current BSONCollectionCatalogEntry object, however does not contain any information about index idents, so we may need to add a way to obtain both the catalog entry and the idxIdent field with one DurableCatalog function call.

See the catalog arch guide for an example of the durable catalog format