rtsp-stream: improve handling of sequence numbers

Change so that gst_rtsp_stream_get_current_seqnum() returns the sequence
number of the last buffer pulled from the appsink instead of the
sequence number in the payloader. If no buffer have been pulled from the
appsink (e.g. in the case of UDP) then the sequence number in the
payloader is used instead.

This gives a more accurate reading of the current sequence number and
ensures that there is no gap in sequence numbers following
PLAY-PAUSE-PLAY when using suspend mode GST_RTSP_SUSPEND_MODE_RESET,
which would be the case without this change.
24 jobs for !122 with rtsp-stream-Improve-handling-of-sequence-numbers in 28 minutes and 11 seconds (queued for 3 seconds)
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