The old free list implementation used objects in FIFO style. This is
bad because it tries keep all the objects ever used alive and in
memory. This minimizes the changes that an allocated object is already
in cache. When there is shortage of physical memory this may also
increase change that newly allocated object is swapped out. LIFO
(e.g. stack) style free list should help these issues. Like the old
one the new implementation is also lock free. This version (v2.1) of
the patch has a potential weakness fixed. The previous version (2.0)
did segfault when popping from empty flist, this does not.