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.

--- a/src/core/ngx_slab.h
+++ b/src/core/ngx_slab.h
@@ -29,6 +29,7 @@ typedef struct {
     size_t            min_shift;
 
     ngx_slab_page_t  *pages;
+    ngx_slab_page_t  *last;
     ngx_slab_page_t   free;
 
     u_char           *start;