fix: use-after-free in tcp_conn_destroy + throttle consumer_ack

tcp_conn_destroy: set read/write queue callbacks to NULL before
queue_free to stop deferred autofetch (drain_cb on freed rc).

write_on_get_cb: throttle consumer_ack to every 8th write_queue
dequeue instead of every single one (was 30+ ACK/sec).

ack_batch field in tcp_proxy_server_conn.

src/proxy/tcp_proxy_server.h

@@ -36,6 +36,7 @@ struct tcp_proxy_server_conn {
     void*    close_timer;      // таймер повтора CLOSE/ERROR
     int      close_backoff;    // backoff: 50..5000 tb (5ms..500ms)
     void*    diag_timer;       // 1-секундный таймер диагностики
+    uint8_t  ack_batch;        // счётчик для throttled consumer_ack (каждые 8 записей)
 
     struct queue_waiter_handle pause_waiter;
     struct UASYNC* ua;