Introduce MetadataDocument.UpdatedBlockIndex

[moreal]

I want to discuss about adding MetadataDocument.UpdatedBlockIndex field to record the latest block index when updated the collection. It will be helpful when caching queried data from database.

https://github.com/planetarium/mimir/blob/bb31e73aa15d0e77f5604d5a127b0ce3106927f3/Mimir.MongoDB/Bson/MetadataDocument.cs

Without UpdatedBlockIndex, users cannot know the collection has changed or not. So they should assume the collection has changed when LatestBlockIndex was changed. But with UpdatedBlockIndex, when UpdatedBlockIndex has not changed, they can assume the collection has not changed though LatestBlockIndex has changed.