After discussion with sue.loverso and keith.smith, we have adjusted our model for the interaction between WT and the storage source module. The WT_STORAGE_SOURCE will no longer need to track what files to flush to the cloud - the flush API should be adjusted to copy a single file to the cloud.  Also in the WT_STORAGE_SOURCE API (and hence in the local_source implementation), we'll want some sort of "migrate" or "move_to_cache" method that takes a WT file that was used in a flush API and moves it into the cache.  This will avoid the reopening the file in the block manager.