vchoi_fork

Rename the deferred-free queues.  There are two of them, and the names I used
for them did not help me keep track of what they were for.

For brevity, I will call a deferred free record a "defree" in the code.

The deferred_free_queue_t becomes a lower_defree_queue_t, and each record on
the queue becomes a lower_defree_t.  A lower_defree_t tracks one deferred free
on the lower VFD---that is, the one under the SWMR VFD.

The old_image_queue_t becomes a shadow_defree_queue_t, and a record therein is
a shadow_defree_t.  A shadow_defree_t tracks one deferred free on the shadow
file.