flvmux: Correct metadata handling in file and stream mode
flvmux: Correct metadata handling in file and stream mode

In file mode, only push one onMetaData at the start of the stream.

In stream mode, always push complete onMetaData. They get replaced, not
merged.

https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/merge_requests/418

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