The inode cache was keeping these alive forever. Added a cute little magic
trick to Retainable that calls T::one_retain_left() when the retain count
is decremented to 1.
Use a little template magic to have Retainable::release() call out to
T::will_be_destroyed() if such a function exists before actually calling
the destructor. This gives us full access to virtual functions in the
pre-destruction code.
