Production-location for Memoizer cache files

This removes the last location with /tmp/memo hard-coded.
Along with a change to Bio-Formats, caching will only
occur when a directory is explicitly set. With the current
setup, this means that caching does *not* occur during:

 * client-side import
 * server-side import
 * any where in Bio-Formats natively

The only time memoization should occur is in the ome.io
(i.e. romio) package during rendering and pixel reading.