rtpjitterbuffer: pause timer when PAUSED
rtpjitterbuffer: pause timer when PAUSED

Also pause the timer when we go to the PAUSED state. It is possible that we
don't have a clock or base-time in PAUSED to perform the timeouts.

jhodapp/gst-plugins-good,jhodapp/gst-plugins-good,jhodapp/gst-plugins-good,jhodapp/gst-plugins-good