pad: Fix race condition causing the same probe to be called multiple times
pad: Fix race condition causing the same probe to be called multiple times

Probes were remembering a cookie that was used to check if the probe was
already called this time before the probes list changed. However the
same probes could've been called by another thread in between and thus
gotten a new cookie, and would then be called a second time.

https://bugzilla.gnome.org/show_bug.cgi?id=795987

StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer