Make a separate cookie to detect chancges in the list of probes and keeping
track of what hooks have been invoked yet.
Remove the requirement to have probes on srcpads in push mode and sinkpads in
pull mode.
Add some more debug.
Keep track of what callbacks got executed. If no callback is called and we are a
blocking pad, let the item pass. This allows you to block pads on selected
items only.
Explicitly have an UPSTREAM and DOWNSTREAM PadProbeType. This allows you to only
block the pad on upstream or downstream items.
Add convenience macros to only block on downstream/upstream items.