More cleanup.  Notable:

- Moved the logger object to module scope.  There's virtually
  no logging of any kind done here, and I suspect that's partly
  because the logging object was clumsy to get at.

- Figured out what the code actually does when the cache size
  asked for doesn't match the actual size of a pre-existing
  cache file.  It apparently wanted to ignore the requested
  size then.  It probably shouldn't, but leaving that for later.
  Still, it left its own idea of the file size out of synch
  with the actual file size, and that was a bug.  For now,
  logged a warning when this happens, and changed the code to
  respect the actual file size.