Core: slab allocator free pages defragmentation. Large allocations from a slab pool result in free page blocks being fragmented, eventually leading to a situation when no further allocation larger than a page size are possible from the pool. While this isn't a problem for nginx itself, it is known to be bad for various 3rd party modules. Fix is to merge adjacent blocks of free pages in the ngx_slab_free_pages() function. Prodded by Wandenberg Peixoto and Yichun Zhang.