When initializing Pad functions in `Pad{Src,Sink}`, we downgrade the
`Pad{Src,Sink}` and upgrade it when necessary. This was implemented
to avoid reference cycles:
`gst::Pad` -> pad function -> `Pad{Src,Sink}` -> `gst::Pad`.
Since `Pad{Src,Sink}` reset the pad functions when dropping, there is
no cycles, so we can use an `Arc<Pad{Src,Sink}>` in the pad functions,
thus saving an `upgrade`.
This is similar to what the standard jitterbuffer does, and
is necessary to avoid deadlocks with the rtpbin session lock,
if the user calls onto any API that requires us to take the
state lock at the wrong time (eg setting the latency property,
clearing the pt map)
They're actually used as a hackish intrusive list around GQueue so using
a different allocator is rather dangerous. Or rather, this is generally
dangerous and shouldn't be done but ...
Also make sure to free items manually in finalize() so that any
contained buffers can also be unreffed.
We don't want to run it every time a strong reference is dropped but
only at the very end. Otherwise dropping the socket stream will cause a
panic because the socket itself is still running.
This can only happen if the receiver is dropped, which only happens when
the task is stopped. As such, Flushing should be returned instead of
panicking.
