uv3 delta serializing- handle case where delta is for non-reachable FS

In version 3 FSs may be unreachable (if not indexed, and if no other FS that is reachable references them). Serialization only serializes the reachable FSs; it can't find the others.

It's possible that some modifications are done to FSs which subsequently are unreachable. When serializing, these modifications can't be found in the set of known FSs being serialized. Handle this case by skipping serialization of those modifications.