libubus: always defer processing incoming invoke/unsubscribe/notify if there is a request pending

This fixes recursion problems on config reload in netifd and simplifies
application handling of requests

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
libubus: fix deferring invoke processing for non-uloop usage

Signed-off-by: Felix Fietkau <nbd@openwrt.org>