appsrc/appsink: Make setting/replacing callbacks thread-safe

appsrc/appsink: Make setting/replacing callbacks thread-safe

Previously we would simply use them without any locking at all, while
using the object lock for setting them. Nothing prevented new callbacks
to be set in the meantime, potentially calling a callback with already
freed user_data.

To prevent this move the callbacks into a reference counted struct and
use the appsrc/appsink mutex to protect access to it, which is used in
all functions calling the callbacks already anyway.

Fixes https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/issues/729