The timestamp field in CollectionType and ShardCollectionType IDL was added in 5.0 (SERVER-52768). Because it previously didn't exist in 4.4, it was marked as 'optional: true', so that we can load the collections that were already present in 4.4.

When upgrading to 5.0, the timestamp field will be added to the collections that didn't have it. This ensures that when upgrading to 5.1, we are guaranteed that all collections have a timestamp.

This ticket is to change the timestamp field in CollectionType and ShardCollectionType IDL to be required (optional: false), once 5.0 has branched out