Improve the performance of the slab allocator

Improve the performance of the slab allocator

This patch makes a couple of changes which testing shows to improve
performance.

Firstly the slabs are now sorted such that the most occupied slab is on
the head of the list. This should lead to a smaller number of slabs, each
with a higher occupancy, which reduces the overall amount of memory
consumed and the number of 'Ctor' invocations (NB: a Ctor may also
allocate other memory).

Secondly, rather than destroying a slab as soon as its occupancy falls to
zero, defer freeing to the re-introduced monitor thread. This takes freeing
(and associated invocations of Dtor) off the performance critical paths
 and also allows an empty slab to be re-used, avoiding calls to
CacheCreateSlab() (and hence more Ctor invocations).

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>