splitmuxsink: Reset cleanly for reuse
splitmuxsink: Reset cleanly for reuse

Reset the splitmuxsink completely when changing states so that
it can be reused.

Fixes https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/issues/1241

GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good