threadshare: save upgrade in Pad functions
threadshare: save upgrade in Pad functions

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`.

sdroege/rsplugin,sdroege/rsplugin