resetVidMap trims the cache chain by storing nil into a node's cache
pointer once it ages past vidMapCacheSize. A lookup that missed in its
own map and only then loaded that pointer could find the link already
severed, reporting "not found" for a volume that stayed resolvable the
whole time.
Load the link first, while it is still guaranteed live. A published
vidMap's cache pointer only ever goes from its ancestor to nil, so
reading it earlier can never yield staler history.
Chris Lu
7fb36025b3