Only try to update the timstamps of blobs which exist in FMB

Description

This MR makes a small optimization to missing_blobs to reduce the number of digests whose timestamps are refreshed. Previously we would try and refresh the timestamps of ALL blobs passed to missing_blobs, regardless of whether they exist or not. Since we already know which digests do exist via _bulk_select_digests, we can pass only those digests to _bulk_refresh_timestamps instead.

It ended up being more code than expected because what _bulk_select_digests returns can't be passed directly to _bulk_refresh_timestamps, so we need to do a bit of extra work to get the Digests which do exist as well as the ones which don't.