The main class is now CacheFSMixin, a mixin class that can add caching to
any FS implementation.  It operates explicitly on a PathMap of recent 
info dicts, making it more efficient and more robust than the old CacheFS.

The CacheFS wrapper class is still there, but it's a trivial class that
just mixes CacheFSMixin into the base WrapFS class.

TahoeFS has also been modified to use the mixin architecture.