__wt_block_checkpoint_last is used (if needed) to find the most recent valid checkpoint during table import.  It scans the entire file looking for checkpoint roots and returns the most recent one. 

This need to be extended to support tiered tables. This means scanning objects that may be part of the table. We should be able to take advantage of the write-once nature of objects and scan them in reverse order, ending the search once we find an object with a checkpoint root.