Stop maintaining list of complete slabs

The complete slab list serves no purpose.

Signed-off-by: Richard Yao <ryao@cs.stonybrook.edu>
Replace partial slab list with rbtree

This permits us to maintain a strict ordering of fullest to emptiest slabs
based on the slab reference count. This reduces internal fragmentation
in the slabs by ensuring that we do not allocate objects from slabs that
could potentially be freed.

Signed-off-by: Richard Yao <ryao@cs.stonybrook.edu>