Add flag to run buddyfs against an in-memory KV store instead of
buddystore.
+ Much easier to develop FS changes.
Fix multiple issues:
+ Previously, files and directories could not be accessed once their
  cached handles went out of cache. This was due to a bug in Lookup,
  where we were returning File/Dir instead of *File/*Dir.
+ Fix handling size updates (shrink/expand), move logic to setSize.