Node.js: implementing output message drain using SHM_ACK feature.

ServerResponse.write() method tries to write data buffer using libunit
and stores buffers to write in a Server-wide output queue, which is
processed in response to SHM_ACK message from router.

As a side effect 'drain' event implemented and socket.writable flag
reflect current state.

src/nodejs/unit-http/unit.h

@@ -36,6 +36,9 @@ private:
     static void close_handler_cb(nxt_unit_request_info_t *req);
     void close_handler(nxt_unit_request_info_t *req);
 
+    static void shm_ack_handler_cb(nxt_unit_ctx_t *ctx);
+    void shm_ack_handler(nxt_unit_ctx_t *ctx);
+
     static int add_port(nxt_unit_ctx_t *ctx, nxt_unit_port_t *port);
     static void remove_port(nxt_unit_ctx_t *ctx, nxt_unit_port_id_t *port_id);