invalidated objects.

Waaa.  I'll probably need to add a separate lock file.

methods did setattrs.

tries to minimize the size of the cache.

"volatile" attributes (starting with _v_) to cause database writes.

class, we save the class mosule and class name.  This allows us to
handle import errors ourself.

Added method to perform incremental gc.

Changed incremental collection effort algorithm to be based on
difference between actual and target size, rather than ration.

constructor when deactivating.