rtpssrcdemux: Forward serialized events to all pads

While forwarding serialized event, we use gst_pad_forward() function.
In the forward callback (GstPadForwardFunction) we always return
TRUE. Returning true there will stop the dispatching procedure. As a
side effect, only one events is receiving the events. This breaks
when sending EOS from the applicaiton, it also breaks the latency
tracer.
3 jobs for master in 7 minutes and 22 seconds (queued for 1 second)
Status Name Job ID Coverage
  Preparation
passed manifest #48912

00:00:19

 
  Build
passed build fedora x86_64 #48913

00:02:08

 
  Full Builds
passed build android arm64 api28 #48914

00:04:54