tempfiles do *not* perpetually accumulate.  (TemporaryFile reliably is
removed when the program is done with it, under both windows and
unix.)

ClientCache.__init__(): Set up the first TemporaryFile, and set both
self._p file names to None, to signify use of temp files.

ClientCache.checkSize(): Switch over to other persistent cache file if
self._p has names, or a new TemporaryFile if they're None.

(I included some comments to the code i touched, to ease the passage
of anyone else who winds up on that path...)