DDL operations present edge cases when performing checkpoint validation:

1. If a collection is created and dropped within the same checkpoint, it is absent from the primary's checkpoint, and the standby should not expect to see an entry for that collection.
2. If a collection is created and dropped across checkpoints, the collection is present in the primary's checkpoint as a container delete, and the standby should expect to see that collection in the checkpoint.

This is the motivation for the type definition

using CheckpointContents = absl::flat_hash_map<UUID, boost::optional<CollectionReplicatedMetadata>>;

boost::none represents a collection has been dropped.