Skip lookaside reads for checkpoints without a timestamp

If checkpoint visits a page and every update on the page was visible but older updates have to be kept for reads, we should be able to evict using the lookaside table and track that the page is "clean" so that subsequent checkpoints don't need to visit it.

This is a common case in MongoDB with append-only tables when the oldest_timestamp doesn't move forward (e.g., because replication's majority commit point lags).