pool: minor fixes (#1092)
pool: minor fixes (#1092)

* Remove pool item constructors/destructors (they are never used).
* pool_alloc: when we allocate a new slab, we have to remove it from the empty list.
* destroy_slabs: set pg->slab to NULL before releasing slab's pages in kmem_free, and update pool's statistics.
cahirwpz/mimiker,cahirwpz/mimiker,cahirwpz/mimiker,cahirwpz/mimiker