Reengineered the double-linked lists for inmem objects, and now another

Reengineered the double-linked lists for inmem objects, and now another
for all objects.  The 'dlink_node' structures are embedded into
StoreEntry and MemObject.  storeGetMemSpace() and storeGetSwapSpace()
start at the tail of the lists and start deleting their way up.
In storeLockObject() we move a StoreEntry to the top of the list
if the lock_count was zero.