Completion-based IO plugins leak memory and sockets

Configuration monitor (HTTP) does not use aspend or anything else to signal lcb_t about pending operations, so it is not possible to wait for all operations to complete before calling lcb_destroy(). With completion IO, we cannot just release resources (memory and sockets) while we have pending operations, or the callback associated with request might write to deallocated memory. But also we cannot cancel requests as the IO API for completion-based plugins does not expose function to cancel pending requests.