Make a helper function check_sticky to check and push pending sticky events.
Move the handling of the result of pushing the sticky event inside the
push_event function, we need to mark the event as received when it was pushed
correctly.
Move the sticky events code outside of gst_pad_push_event_unchecked and
make it purely handle sending the event to the peer.
when pushing a sticky event, first store it on the pad. Then check and push any
pending sticky events when we get a serialized or sticky event on a srcpad. This
fixes the issue where sticky events are not pushed when an event is pushed.