add delay to purg'ing; call collect_retired every N generic allocs

include/mimalloc/types.h

@@ -409,6 +409,7 @@ struct mi_heap_s {
   size_t                page_retired_max;                    // largest retired index into the `pages` array.
   mi_heap_t*            next;                                // list of heaps per thread
   mi_memid_t            memid;                               // provenance of the heap struct itseft (meta or os)
+  long                  generic_count;
   long                  full_page_retain;                    // how many full pages can be retained per queue (before abondoning them)
   bool                  allow_page_reclaim;                  // `true` if this heap should not reclaim abandoned pages
   bool                  allow_page_abandon;                  // `true` if this heap can abandon pages to reduce memory footprint