Request-app link introduced to prevent mp destroy for penging requests.

nxt_req_conn_link_t still used for lookup connection by request id.
New nxt_req_app_link_t (ra) allocated from conn->mem_pool using mp_retain().
ra stored in app->requests if there is no free worker to process request.

src/nxt_port.h

@@ -74,6 +74,7 @@ struct nxt_port_s {
     uint32_t            max_size;
     /* Maximum interleave of message parts. */
     uint32_t            max_share;
+    uint32_t            app_req_id;
 
     nxt_port_handler_t  handler;
     nxt_port_handler_t  *data;