* stop caching most_recent_size in dirnode, rely upon backing filenode for it
* start caching most_recent_size in MutableFileNode
* return None when you don't know, not "?"
* only render None as "?" in the web "more info" page
* add get_size/get_current_size to UnknownNode
The idea is that future versions of Tahoe will add new URI types that this
version won't recognize, but might store them in directories that we *can*
read. We should handle these "objects from the future" as best we can.
Previous releases of Tahoe would just explode. With this change, we'll
continue to be able to work with everything else in the directory.
The code change is to wrap anything we don't recognize as an UnknownNode
instance (as opposed to a FileNode or DirectoryNode). Then webapi knows how
to render these (mostly by leaving fields blank), deep-check knows to skip
over them, deep-stats counts them in "count-unknown". You can rename and
delete these things, but you can't add new ones (because we wouldn't know how
to generate a readcap to put into the dirnode's rocap slot, and because this
lets us catch typos better).
