rtpgstpay: delay events until first buffer so they can be timestamped

Submitted by Sebastian Rasmussen

Link to original bug (#719587)

Description

While developing a patch (concerning RTP-Info header) for gst-rtsp-server I stumbled upon an rtspserver test using rtpgstpay where my newly introduced code expected the first payloaded buffer in each media stream to have a timestamp. Unfortunately rtpgstpay sent an non-timestamped event as its first RTP packet so my code failed.

Now, there is no reason to not timestamp the events packaged into RTP packets so therefore I opted to delay the events until the first buffer arrives and give both events and buffer the same timestamp.

Finally there was no unit test for rtpgstpay so I created a silly one that at least exercise the rtpgstpay a little bit.