Disallow direct use of g.memcache.

Caches should always go through the appropriate cache chain. Making the direct
CMemcache object a local var should prevent abuse in the future.
cache: Add a stats setter for TransitionalCache.
cache: Add caches property to TransitionalCache.

This attribute is introspected by some hooks.
TransitionalCache: return properly from set_fn.

This is causing e.g. `incr`s that check the return value to fail as the
return value is always `None`.