Mem pool cleanup introduced.

Used for connection mem pool cleanup, which can be used by buffers.
Used for port mem pool to safely destroy linked process.

src/nxt_mp.h

@@ -109,6 +109,10 @@ NXT_EXPORT void *nxt_mp_zget(nxt_mp_t *mp, size_t size)
     NXT_MALLOC_LIKE;
 
 
+NXT_EXPORT nxt_int_t nxt_mp_cleanup(nxt_mp_t *mp, nxt_work_handler_t handler,
+    nxt_task_t *task, void *obj, void *data);
+
+
 NXT_EXPORT void nxt_mp_thread_adopt(nxt_mp_t *mp);
 
 #endif /* _NXT_MP_H_INCLUDED_ */