QuickJS: fix unhandled rejection tracking in reused contexts

QuickJS: fix unhandled rejection tracking in reused contexts

ngx_qjs_destroy() unregisters the host promise rejection tracker on the
runtime, but ngx_qjs_clone() did not re-register it when a context was
popped from the reuse queue.  Every request served by a reused context
then silently dropped unhandled rejections instead of reporting them,
which also defeated any caller relying on ctx->rejected_promises.