Use site/vfs prefix for 404 parent response caching

[themightychris]

Caching 404 from parent sites currently uses a global cache key namespace to gain some efficiency from multiple sites on the same server sharing the status. This however puts the cache key outside the scope of tools for clearing the VFS or application cache for a site, requiring a full PHP restart to clear it.

Prefix the cache key the same way that file/collection caches are keyed, so that the "repair filesystem" task's clear cache action will clear out 404 caches too